1) Don't know how to organize themselves
2) Can't digest lots of info
3) Underestimate the focus required
4) Goof off during training because they have always been the life of the party (why stop now?)
5) Lack of commitment
6) Man got no skills
7) Expect it to be spoon fed (If "Daddy" paid for you to go to an expensive flight school, pay special attention to this one.)

It is rare for the person who is it giving their all to fail.

Former B727 ground instructor/check airman

Just $0.02