How to handle a dog quit hunting?

It could be a number of things, everything mentioned already makes good sense and is a good place to start. My question is, is that a known trait for that line of dogs or the yard it came off of or one of his ancestors, my reason in asking is, I once hunted with a group of me who had a family of dogs they had bred up over the years that were all known to have a slump around 2.5 years, it usually lasted around 3 months, I always heard them mention it, well a man in the group give his son a dog that was very good at a young age I hunted with the sucker and was impressed, about 6 months later I went hunting with them and you couldn't have give me the dog, well the son kept complaining about him that he had been like that for a month, one of the men told him to sit him up for a couple of months, well instead of doing what they told him he give the dog to some people for a yard dog, about 6 months later my friend gets word of the dog being a pet so he calls the guy who give him away and ask he if here ok with him trying to get the dog from the people, he agreed and long story short he went and got the dog and he wound up making a phenomenal dog, ole boy would always just shake his head and kick himself in the butt every time he hunted with him, so I guess in all that rambling might want to look into his background if you don't already know it.