i think you are on track , the heat and dense cover.
x2, The hogs look for the thickest cover when chased. They run through the thickest brush to put distance between them and the dogs, then they hide in the biggest briar patch they can find. They learn quickly that the dogs will lose them and they will make each hunt the same routine that worked before... The Hot weather doesn't help.
It sounds to me like you have a good dog...