I too think that nobody is beyond learning to fly, hell NASA taught monkeys to fly sims. I have seen a few people go through the same things that other's here are saying. The key is to find what works for a particular student. One person at my club was having trouble learning to fly. It was'nt until someone started talking to him about model flying and they found that this guy didn't understand what was happening or what to expect when he moved the sticks. I.E he needed to be shown (on the ground) That the aileron and rudder are not the same thing, that a change in power will change the pitch attitude of the plane etc. Once he understood what was going on he progressed at a more rapid rate.
You could try to get them to fly a sim for more practice too.
Think of these problem students as a challenge, don't give up on them. They have probably spent a large sum of money to give this hobby a go.