She sounds like maybe she's bored, or, she has trouble organizing her work. At this age it seems like a lot of kids get overwhelmed by just starting a project - it seems like so much - and also, it's often very boring. For children who already know how to spell all the spelling words, writing them 5 times each is so tedious.
Can you keep up with her on a daily basis? Each day, can she bring home the work she didn't complete in school to do for homework? You'd get a sense then for why she is balking.
Does she have friends she looks forward to seeing at school?
You are right her work she gets back tells me she is capable of much more but is not too keen on doing it.
sona, is school too hard for her and she's giving up trying? Or is it so easy that she finds it boring?
Is she in a group of girlfriends who are all more interested in socializing at school than doing school work?
When you see the work she brings home does it seem like she is capable of doing it but somehow doesn't?