"Trout will often pull a fly into their mouth by clearing water through their gills. In other words the "suck" the fly into their mouth. If you are fishing down stream and are tight to the fly you'll feel the "bump" of the fish trying to take the fly but since the fly can't move you get a short strike. While swinging a streamer, soft hackle, or a nymph try raising your rod tip to allow some slack into the line. This will allow the fish to pull the fly in and you can get a hook-up."

Thank you John, that may be the best advice I've heard all year as it's been a particular problem for me lately.