I had a similar experience this past summer.  Having had great success in previous Augusts down deep, this summer I was fishing much shallower not only for Walleye but also trout and Pike.  I didn't catch any monster Eyes but I brought in 2 Pike over 40" and a couple over 36".  As for trout, last summer I was down around 60' and catching quite a few in the 6 lb range.  This summer I wasn't more than 25 or 30 feet for the same amount and size.  According to my Fish Hawk sensor the water was colder this summer, reaching 50 degrees at around 25 feet in  most of the lakes I was in (WCPP.)  I don't know why the water was so much colder, certainly the 3 weeks I was there it was a virtual heat wave.