Sure, a kid might melt down because they can't do what they're being asked. Or:
1) They just think they can't do it.
Then the skill they can't do is risk trying, or accurately assess their own skills.
2) They don't want to do it, and melting down is a deliberate act, because it has been proven to be a useful avoidance strategy in the past.
I think this is the one that Ross Greene thinks just doesn't happen. I'm not 100% sure he's right about that, but he has a pretty impressive track record with kids in juvenile detention facilities and other situations where you would think that there would be a reasonable incidence of this.
3) There's a physical problem involved (tired/sick/injured/uncomfortable/etc.).
The the issue is not "can't do it" generally, but "can't do it when I'm tired/sick/injured/uncomfortable," but they still can't do it.
4) There's a negative emotion involved (fear/anger/boredom/resentment/embarassment/etc.).
See #3.
