I have noticed over the years that using smaller nymphs/dries than what is expected to hatch on heavily fished water is very productive. When I first started out years ago, I would strickly stick to exactly what was on the water and under the rocks. I won’t argue it produced fish, but when the fish become educated or so called “received their docterate degree in presentations” I found that reducing your fly size will usually produce more hook ups. Yes, matching the hatch is important, but more often than not a fish will be more willing to take a smaller sized nymph/dry than it is to take a size bigger. Bottom line, if you are not catching the amount of fish desired with your nymph/fly and that is what is hatching or is in the stream/river, try switching to a size or two smaller, for instance if the water is known for a size 18 pheasant tail try a size 20 or 22 and it is a good bet you will be more successful.