When you are fishing the dries to rising fish, a good place to start problem solving if you aren't getting action is to make a quick evaluation of your presentation. As a preventative measure, to avoid presentation problems, always make sure you are using a good tapered leader with the right tippet to balance well with your fly and get the right presentation. It is easy to get into a rush to fish and not properly balance things out when rigging up. As a general rule the larger the fly the larger the tippet. Big Foam hoppers will twist and spin 4x leaders, 3x tippet will turn over most wind resistant flies and insure you don't lose that trophy trout that may hammer your dry. When fishing smaller dries, say size 14-18 you must match your tippet size to the water you are fishing. If I am on the Missouri River fishing trico's to finicky trout I have at least a 12 foot leader and fishing 5x. Yes I said 5x. Many clients talk to me about fishing 8x and hooking fish but rarely landing them. Here in the Rockies whether the fish are not quite as leader shy or we can get away with it I do not own a spool of anything lighter than 5x. In fact I have been using 1x or 2x here lately with the salmonfly hatch. If I have the right fly on and the proper presentation I know the fish will eat it and I want to be able to land them.
If you feel that your cast and presentation are even slightly off, then the fish will mostly likely take notice as well. Some minor adjustments with your leader/tippet can make a tremendous difference. Also, if you have experimented with a number of flies and found one that is getting looks, but not takes, try the same or similar pattern with minor changes in size and color. The slightest of changes in these areas can sometimes make a world of difference.
When fishing below the surface, whether under an indicator or with a dry and dropper combination, it is important to get the flies to the right depth. Fishing under an indicator makes it easy to change the depths you are working. In a lot of instances you want to be fishing the bottom of the river. If you are not catching bottom sometimes then there is a good chance you might not be at the right depth and a quick slide of the indicator will get you where you need to be. Other times, your flies are not getting down fast enough to get you to the right depth as you drift the prime spot. Adding a little weight to find the right amount will help get you where you need to be. When working under a dry fly, depth still needs to be taken into consideration. Trying flies on different lengths of tippet trailed off the dry to find out where the fish are feeding can often produce more consistently than the lead fly once you find the proper pattern and position. Experimentation with depth, weight, and flies will help you locate the fish and key you in to what flies they are interested in. When you aren't producing like you should be on the water change up a few of the little things to see what is working and get yourself back into some fish.
Taking some of these issues into consideration can significantly impact your fishing in a positive way. When you are on the water take the time to check some minor things. True, these are all small things to consider, but if you take care of the little things, the big things generally take care of themselves.