This is almost impossible to answer because there are so many reasons that it could be. But I've been hiring manager and looked through thousands of resumes/CVs and hired about 50-60 engineers.

1. Far under-qualified. People apply for senior positions with literally 18 months of experience ALL THE TIME. Seriously... ALL THE TIME.

2. Too many technologies on your CV. A lot of resumes will just list every language or technology on the planet. Its a shotgun technique to just list everything and hope something sticks. The problem with this is that you can't be good at everything. When you say you are confident or even expert in 30 different languages, then it just tells me I can't trust anything on your resume.

3. Titles that don't make sense. Your titles at companies should be standard. Don't put "Javascript Ninja Pro II". Wtf is that? Just put "Software Engineer II - Javascript". Also, in the vein of job titles that don't make sense is someone who worked as a junior engineer for 6 months, and then their next job is as a Senior level engineer. This is scary common, seriously. You're either lying or delusional, either way I'm moving to the next CV.

4. Colorful resumes. Resumes should be text documents. A colored line below a header or little highlights are fine, but its not a graphics design job. Cutesy resumes just make me worried that someone is overcompensating or hasn't been in the industry enough. A lot of cute resumes have tons of whitespace and design to take up space and make a light resume fill fuller. These resumes are super common with bootcamp grads. Maybe bootcamps are selling resume templates as art projects. Don't know, don't care. Usually a well qualified engineer will want a text dense resume so they have more space to brag about all the amazing things they have accomplished. If youre using space for fancy designs then you probably don't have enough content on your resume. It's essentially wasted opportunity.

5. Resumes with Personal Photos. DO NOT EVERY SUBMIT A RESUME WITH YOUR PHOTO ON IT. I have a rule that I automatically reject any resume with someone's photo. It's too risky. I can either get accused of favoring a certain race/gender/ethnicity/etc or being prejudiced against them. So instead I automatically reject any resume with a personal photo. You can be the worlds best engineer and you won't get a call back from me because you added a personal photo on your resume. A lot of companies have this rule too. So if you are adding your photo to your resume you are doing far more harm than good to yourself.

6. Too long. Most resumes should be 2 pages. If you are just getting into the industry and only have < 3 jobs then you should use 1 page. If you are a veteran than use 3 pages. Never go above 3 pages. Anything above 3 pages will get cut. Three pages is enough to give even a staff-level engineer plenty of space to talk about their accomplishments. Anything more than that is an autobiography and I don't have time for it. Learn to cut fluff and give me your greatest hits on a resume. You are just trying to get an interview. You'll have HOURS in far-too-long interview process to give us your verbal autobiography. Save the greatest hits for the resume/CV.

7. Pattern of short job history. Some companies might not care. But if you have a consistent pattern of only working somewhere for 6 months (again, very common) then it tells me you are just job hoping. I know the saying that this is the best way to get a raise in tech. But if youve had 4 jobs in 2 years then I won't waste my time. It will take you 6 months to even start to provide value at the company, and i don't want to be replacing your position twice a year. Im fine with a single job that has a short stint. Just as long as its not a pattern of behavior. Its really common to get resumes with engineers that have 6-8 jobs and only 3 years of experience (Its very common). I'm not delusional enough to believe that our company will be the one you stick around with. So I move on from this. As long as you have an average pattern of at least a year with jobs, then you're fine. If you worked contract, this is different and you should list that in the job description.

8. No good bullet points. With each job you should list things you accomplished. "Built the website using Javascript and React" is not an accomplishment. If you don't have specific accomplishments to list and everything about each job is superficial, then I will assume your contributions at the company were superficial.

9. No Progress on Resume. Your resume should tell a story. If I look at someone who has worked somewhere for 3 years and didn't get a title change or promotion during that time, then I get worried that they aren't providing good quality work. Good engineers get promotions. I'd expect to see that story as I read down your resume. If you look stagnant then the resume will get tossed.

10. No Clear Direction. I want to see that someone is looking for a specific type of job or they are pointing their career in a certain direction. This is visible by the content on the resume being focused in one area or direction and the pattern of jobs taken. I see a lot of resumes where people take a job as an ETL Engineer for 18 months. Then they are a full stack engineer for 12 months. Then they get into sysadmin work for 12 months and now apply for Backend Java. If you have legit reasons for meandering in your career then it should be in your cover letter or explained in 1-2 brief sentences at the top.

Stuff that doesn't matter.

- Your home address, not sure why people put this on there. I'm not going to knock on your door to schedule an interview. I'll get your address if/when you're hired.

- Photos. Its a engineering or tech job, not a modeling gig. See #5, these automatically get cut by most companies for risk of subconscious bias.

- Mission Statement. Sure put a sentence or two if you want. But "Get a job in software engineering" is not a mission. If that's all you have to say then leave it off. Mission statement is not going to have any impact positive or negative on you getting a job. So why bother having it there.

- Jokes. You're not going to get hired because of a joke and its really hard to tell whether stuff is a joke or you're just crazy when its on a resume. You're not lightening the mood, you are confusing the hiring manager. When a hiring manager gets confused while looking at the resume, the resume goes in the trash. So jokes are only going to hurt you.

- Typos. I don't immediately dismiss resumes for a single typo. But more than 3+ and I probably will. Its easy enough to use spell check. If there are typos it is because you rushed the resume-building, have no attention to detail, or just don't care. One or two typos are fine. But a pattern is concerning.