What is the hardest area on the body for you to teach on how to train?

[TheTransMan]

For some reason for me, it's the abdominals. I give cues like curl the pelvis under and leave it there, put your feet up higher on the ball, breathe out when going, etc. Anytime I give a cue, they never due it. They wonder why their back hurts, so I explain, aaaaaaaaand they keep doing the same things. Proper spinal flexion movements are not all that difficult for the most part, but people treat it like a whole new science. I honestly don't know what it is with some people -_-

What about you guys?