Use the fan casting technique
Most fly fishers have a natural tendency to cast immediately in front of themselves on every spot at which they fish. This means their flies will only be covering a narrow strip of water, and the trout maybe somewhere else.
Instead, try to cover the whole area of water in front of you, including the margins on either side. Imagine a grid or fan pattern drawn on the water, and place your casts from as far left as you can go, all the way around the fan until you’ve covered the area to your right.
Once you’ve gone all of the way across the fan from left to right, start again and re-cover the area you fished a few minutes before, perhaps with a different fly, a different retrieve, or at a different depth. You’ll greatly increase your chances of finding any fish.