I've seen several students not pass OW (and I shouldn't have, at least when I did
). One was so passive, she simply refused to swim (I'm not kidding). One seemed like he had ADD, and couldn't focus long enough to follow directions. He went through, IIRC, THREE different classes with different instructors and didn't go on to his OW dives with any of them. Two teenagers just either didn't do their bookwork, or couldn't retain it, and couldn't pass the exam. They got Scuba Diver certs, and went on to finish up later.
In my limited experience, what keeps folks from passing is either anxiety or lack of focus or application. With more advanced classes, things are different. I didn't pass my first Rec Triox class because I couldn't execute everything expected of us within standards on any given day, but I worked with a different instructor and went on to pass. I didn't pass Cave 2 because of situational awareness issues. But the standards for those types of classes is totally different from that of OW.

In my limited experience, what keeps folks from passing is either anxiety or lack of focus or application. With more advanced classes, things are different. I didn't pass my first Rec Triox class because I couldn't execute everything expected of us within standards on any given day, but I worked with a different instructor and went on to pass. I didn't pass Cave 2 because of situational awareness issues. But the standards for those types of classes is totally different from that of OW.