I may be off-track, but I think I've seen this before - decades ago in myself ;-) She's been underchallenged and disengaged for a while, hasn't she, with the improvement coming just this year at the grade-skip? I would suggest that she has unconsciously developed the habit of skimming over things sort of as a self-defence technique, to protect herself against things that were just too boring, and is now having trouble giving up that technique. I think paradoxically, my impulse would be to give her some seriously challenging work, and see if she can (learn to) do better on that. I bet it'll be easier for her to pay attention to details in the context of a problem that's not boring. Once everyone is satisfied that she can do that, then her inability to do it on easy problems becomes, not unimportant, but at any rate a curiosity rather than an indication of her ability. Hopefully, she'll gradually be able to let go of the technique.
We've just got Descartes' Cove (
http://cty.jhu.edu/ctyonline/cove/index.html ) and I think that has the characteristics I'm thinking of: multistage problems involving several arithmetic steps, so that you have to get them all correct to get the right answer, plus immediate, non-judgemental feedback on whether you have the final answer right or not. Just an example...
