I would first off take a look at the fly. Is there a large tail on it? It could be fish hitting the tail of the fly and not connecting ith the hook. This used to happen often to me when fishing wooly buggers. Shortening the tail helped to increase the hook rate. The other thing I would suggest is to wait an extra second or 2 before setting the hook. See if you can get together with one of the more expreienced fishers on the board. They can offer som great tips if they can see you in action.
Try a few different flies as well. Once the clouser has crossed the pool a few times, it wouldn't hurt to change things up. San Juan Worms, Bead head nymphs and other streamers would be a good start here. Hope that helps a bit.