I started a couple of similar threads, one for each of my DSs. Here are the links in case the replies given might be helpful.
http://giftedissues.davidsongifted.org/BB/ubbthreads.php/topics/96825/1.html

http://giftedissues.davidsongifted....an_he_not_see_the_problem.html#Post87166

DS7 tells us that the reason he doesn't get his work done is because it's too easy and he gets distracted.

We had HUGE issues with DS9 (DYS) with this early this year -- he'd miss whole sections of a test, and end up getting an F. It has gotten better over the course of the year, and I attribute a lot of that to the fact that he is able to test out of the work that he already knows. (Amazing how DS doesn't overlook problems on the pre-tests, when he knows that a good enough grade means he can do more interesting work, huh?) DS9 still struggles with checking his work, but I strongly believe that a lot if the issue can definitely be a function of it being hard to keep a young brain focused on something that is not remotely engaging.

We tried several different tactics with DS9, after brainstorming with the teacher:
1) cover the worksheet/test with another piece of paper, so you only see one problem/question at a time. This seemed to help him focus on what he was doing.
2) At the teacher's suggestion, I created an answer sheet for DS's homework each night, and gave it to him when he'd finished the work, so that he could correct it himself. Then, I went back over it and would tell him if he needed to re-check it again, giving him no indication which or how many problems were wrong. I think the reason this helped was that he saw he was spending A LOT more time on the work by not doing it/checking it right the first time.

Don't know if this helps, but you're definitely not alone around here.