Insurers might decline a life insurance application for a few reasons. Health and lifestyle issues, such as smoking, a high BMI or current health conditions, or hazardous jobs, are just a few where the insurer might see the risk being too great. But different providers have different criteria, which means that if one insurer declines your application another might not.
Even if you have a pre-existing medical condition, which is a disease, illness or injury you had before taking out the policy, you may still be able to get cover. It just might cost you more, and the insurer may ask for more medical information.