Ditto Grinity
The problem is, DS has been getting frustrated with the problem solving enrichment packet. He gets really upset that it doesn't come easily to him and that he doesn't always know the anwer right away, and he'll sometimes get frustrated and cry a bit.
This may also be because the enrichment work is not fun. The fact that he is instructed to do it after he finishes his other work may also be causing the fatigue.
My DD pulls out the old "its too hard" if the work is boring and other times she is just not in the mood to think.
FYI: Masterminds riddle math books are a great enrichment They are fun and challenging.