for our kid (at 5) it was a desire to blend in that was causing it. at first, her teachers never offered her anything appropriate and everything was easy for her, but she could see the other kids had to struggle with the material, so she hid her ability. when this finally came to light, the teachers didn't want to believe it, so DD had to "prove" herself to them. when she did, they offered her "differentiated" work - which wasn't accelerated, there was just more of it and she was expected to complete it faster, and always perfectly. so she decided it was better to not bother.
like mykids so wisely mentions - finding the "turning point" was key. i realized that for DD5, she needed to get back in touch with who she really is - what she really loves - and get comfortable with that again. all the underachieving (and other negative) behaviours were about projecting what she thought her teachers/peers wanted from her. so we're working on that - and (here's the success part for you!) it's starting to come back. suddenly she's not minding being the youngest (by 2-3 years) in all her extra-curricular classes - she's starting to understand that being ready for some things early is not wrong - it's what's normal for her.
