If it's been real hot by you, (5+ days of 90 degrees plus weather) the buck can go sterile... it isn't permenant, but depending on your buck, it can take 2-8 weeks of cooler weather to "get him going again". Unless you can put him some place cooler for a few weeks... I know one breeder that moves all her boys into her basement the end of July so she can start breeding mid August.
The other issue could be the girls... if it's too hot, they just won't breed. Most of my girls miss when it's really hot.
Might be wise to wait a few weeks and try again, especially if it cools down a bit by you. (Breeding early in the morning won't do any good if the buck was exposed to really high temps for days before hand. You may have to just wait him out... or try using a buck that has been kept cooler.)