Can you post a picture of the bush and the leaves? Are the leaves all yellowed out or do they turn yellow but the veins remain green? Are the leaves heavily spotted? Does your soil drain well or is clayish? What were the results of your last soil test (if you have done one)?

Iron chlorosis could cause a yellowish look but it would not be limited to the leaves inside the shrub only.

"They sure aren't suffering lack of water - we've had the wettest May, June, and July on record here in Chicagoland."

Bingo. Too much of a good thing. Better drainage might help, add lots of organic matter in the form of annual mulch and a top-dressing of compost in spring.