There is a temptation is to use long shanked hooks when tying imitations of large insects, like some large species of mayfly or Daddy Longlegs. The weight and stiffness of the hook can cause problems when fish are proving selective.
FANWING MAYFLY SPINNER DRY FLY PATTERNS. Hook size 12 16 - $US each
If you really want to catch trout using a fly rod then you should be on the water fishing just as the sun rises and late afternoon until the sun set. These generally are the optimum times to catch fish that are feeding on the morning or late afternoon/evening hatches of insects. That's not to say you can't catch fish during the more social times of 10am - 4pm using a fishing fly, it's just a lot harder. These time periods vary in different parts of the world because of local climatic conditions, but generally if you find out when your local fish, most commonly feed, that is the time that you should be fishing.
Whilst on a business trip, that had meetings early in the morning, I had planned to go fishing later in the nearby trout. When I got to the water's edge, it was about 3pm. I walked the entire two mile length the river beat I had been given. I carefully peered into every pool, behind each large rock and clump of weeds. The river was fishless. There was no movement to be seen. I had eaten a good lunch and a pint of beer. It was a sunny day so I found some shade under a willow tree and had a sleep. When I woke up about three hours later the River had totally transformed. It was alive with trout feeding on a fall of light-coloured blue dun spinners. I had some fan winged imitations in a flies in my fly box. I had a fantastic evenings fishing. I had to phone my wife and say that the meetings were running late.
Later on that summer I had to go back to the same company for another morning business meeting. Of course I took my fishing gear. It was a sunny day with a cloudless blue sky. The River depth slow as they had not been a lot of recent rainfall. Most of the pools were empty apart from a few minnows. As it got towards evening, I noticed a Brown trout swimming upstream into my fishing beat area. It looks like it was moving from its resting lies to its chosen feeding flies. Then more fish came upstream to join this first trout. They knew something was going to happen. Then the pale watery duns started to trickle down the flow past the hungry mouths of the waiting brownies. I tied on an appropriately sized dry fly, cast upstream and let it float past the lurking hungry trout.
A River can totally change its composition of fish within a few hours. You have to study the water and look for these feeding lies. During the middle of the day they can be empty, but this can drastically change with the onset of the evening change in air temperature and insect hatches. If you want catch trout, then fish the water when they are actively looking for food. Don't go home for an evening meal. Have a big lunch down the pub at noon and get onto the riverbank or in your boat late afternoon to early evening.