Had the same thing happen to me in SW Wisconsin. Strong start to the season in May with plastic frames and queen excluders.
June 1 it all stopped. I swapped new frames for drawn frames with a friend and that kind of spurred them on.
In frustration I pulled the queen excluders in early July, thinking even brood up in the honey supers was better than nothing.
I checked all five hives over the weekend and it looks like every single one of the bees suddenly realized what they are supposed to be doing up in the honey supers.
The goldenrod is blooming and they are bringing in lots of water and pollen as well. All the frames with nectar are just starting to be capped so I figure in two weeks I'll harvest what I get.
Next year I'd say let the queen lay in the first honey super above the brood nest. The workers will draw out the comb and when the brood hatches, checkerboard the super with this comb. They'll put honey in it in no time.
Here's something else: EVERYTHING in SW Wisconsin is two to three weeks early: the apples, tomatoes, melons, etc. and it has been pretty rainy this year...I think that has something to do with it.