Wetnet,
I go through spells where I just can't seem to set the hook into a fish. Sometimes it is just some kind of phase I'm going through. Luckily it wears off. Other times, especially when I'm tired, I tend to over-react and not hesitate a half second before setting the hook. That half second/pause after the strike takes time to pick up and fortunately, it becomes second nature. Also to add to what H-A suggested above, try setting the hook in a direction that puts the hook into the fish. For example, if fishing downstream set to the side instead of up. It will pull the fly into the corner of the fish's mouth instead of out of it.