I fish the Grand a lot and I have found the same thing. Big fish July and early August, now really slow. I was out tonight and caught a small SMB, no where near the size I'm used to. I get afew hits but no smashes on the topwaters. The big fish seem to be there still (big hits, surface feeding) but are on a negative bite. I'll try some of Troutologists tips in October.
P.