When I was first introduced to this approach at presenting flies during obvious surface action I was astounded at how much more productive the subsurface flies were than the dries, and in that particular instance the surface was near boiling with rising fish. Initially, we were casting some "match the hatch" caddis patterns to the risers, but this did not draw any interest from the fish. Next, we did some experimenting with some subsurface patterns and we found "the" pattern they were after. Once we had the right rig the fishing turned on quick. Even though the fish were hitting the surface hard, we were really producing subsurface. That trip was a real learning experience for me and it has really opened up my options when I am getting stumped on the surface. Here are a few tips that can helps you when you are in the midst of some surface action that isn't producing.
First, if you can throw a good hatch matching pattern with a dropper off of it then keep that dry on the line. This is your best bet at having two legitimate chances on the water at once. If the dry pattern you want is to difficult to spot through the naturals or not handling the dropper well you may need to switch up the lead fly to a larger, more visible, or better floating fly. It is still good to choose a fly that has a chance at getting some action and one that is as close to the insects on the water as possible, but it is also not out of the question that an attractor will bring fish up too. Once you have decided to add a second fly, you have to choose which one. Starting with an emerger pattern for the naturals that will get caught in the surface or ride just below is a great place to start. Work with a few patterns and sizes to see what draws attention. Keep your eyes locked on the dry and set the hook with any type of suspicious movement. Also, don't be surprised to get strikes on the dry as well, since this is still a very real possibility. If, after a number of pattern changes there is still no action, choose a nymph that matches the naturals and drop that off of the dry. Continue to experiment with similar patterns until you get on the one that gets the action.
Don't let getting stumped on the surface frustrate you too much during your time fishing. Have the patience and flexibility it takes to experiment with a variety of set-ups to find which one works. We all love catching fish on dries, but when they aren't getting it done, take it below the surface and catch some fish.