I worked with a girl who was constantly late to everything. That affects our schedules and the people who are expecting us to be there. Transportation can also be a huge barrier. Honestly, I prefer to do the driving to the assessment sites sometimes because then it's under my control. When the parents are picking us up, we'll sometimes have to sit around and wait if we finish early.
Some people are their own barriers too-if they are uncooperative or leery about the whole process.
And different disabilities come with different health problems, too. There have been a lot of times where we've had to cancel or reschedule because something's going on related to an individual's disability. Sometimes it seems like those around the individual who should be supporting the process don't take it very seriously. I've had parents tell me that someone couldn't make it to an assessment because he was going to a movie. You can go to a movie any time.