Catching a fish on dry fly must be the pinnacle of fly fishing. The first time you see a trout rise and mouth a perfectly presented dry fly it will add a whole new dimension to this facet of fishing, which differentiates the fine art of fly fishing from other means of bringing fish to the table.
To successfully and consistently catch fish on dry fly requires some additional knowledge and practiced skill that is not required when stripping a streamer or drifting a nymph. Once the appropriate fishing equipment has been selected, the choice fly has been fixed to the tippet and the fly is within striking distance of rising trout, the next skill to master will be setting the hook.
A dry fly is usually tied to sit high in the surface film. The materials used such as spun deer hair and "palmered" hackles can bury the hook which lessens the chance of hooking the trout and does occasionally account for missed fish. However, the largest contributor to missing fish on dry fly is an anglers timing when striking the fish.
When pursuing trout with dry fly there are two distinctly different strikes which must be employed depending on the type of water being fished, namely rivers or lakes:
Striking fish in fast flowing rivers
Trout which live in fast flowing rivers typically hug the stream bottom or lie and wait in an eddy for food to approach them. As there is a limited time for any passing food to be consumed before it is swept downstream, the trout has to act quickly or else risk losing the meal. The fish tends to dart out of its feeding lie, mouth the fly with purpose and close its mouth directly after inhaling the fly (predictably since the turbulent currents could wash the insect from the fishes mouth). These actions in quick succession allow the angler to set the hook directly after seeing the rise. Gently lifting the rod and drawing in the slack line will give the fish enough time to seal its mouth and will be of sufficient force to set the fine wire hook used in dry flies. A whip strike used in bass fishing will make quick work of the tiny hooks and delicate tippets needed for this kind of fly fishing.
Striking fish in slack water
Fish who live in lakes, pools and slower glides in rivers react differently to dry flies than what their siblings do in faster waters. The fish in slower water tend to cruise around looking for their next meal instead of hiding behind boulders away from the current waiting for the food to reach them. This tends to influence their feeding behavior down to the way in which they would mouth a dry fly. Just as the slack water allows the fish to rise up to the fly in their own time, so their ascent and take is more lax and the fish tend to close their mouths only after turning down. To effectively set a dry fly hook for a slack water trout, the same drawing up of slack line by lifting the rod must be employed. However, delaying the strike a second or two after the fish has sipped the fly will allow the fish to seal its mouth and turn down towards the lake bottom. This delay greatly increases the chances of a solid hook-up.