Many fish often feed on or near the bottom. That's just how it is. When those fish are totally keyed in to that particular zone, your fly had better be there or you may come up empty.
That's one of the simplest mistakes anglers can make—not fishing deep enough. You might be in the right spot, using the right fly, and even using the right line/leader combination, but your presentation or simply your patience can keep the fly from reaching that critical strike zone. It's also a very common mistake that I've made in the past and have seen others make as well. Believe me, even just an extra foot or two of depth can make a substantial difference.
One of my favorite spots at a local bass lake is a narrow channel with an adjacent main lake point. Using a floating line, I sometimes start covering that area by casting out, letting the fly sink just a few seconds, then stripping in. The goal is to fish that upper column of water first to see if any active fish are either cruising off the bottom or want to come up to strike the fly. I'll get bit doing that occasionally, but the real success often comes when letting the fly sink longer.
After hitting that upper water column for a few minutes, I fish the exact same water with the exact same fly, just much deeper. Rather than letting the fly get down only a few seconds, I'll wait 15 or 20 seconds before starting the retrieve. The results can be night and day, even in conditions when you'd think any bass that's around would willingly oblige to rise up and hit a fly. I recently fished through this very area, first starting off shallow and picking off just one 12 incher. As explained above, I then started fishing the fly deeper in the EXACT same water and hooked into about a half dozen bass, all 3–4 pounds.
Again, you can be in the the right spot using the right stuff, but really ask yourself the questions: "Do I need to fish deeper, or am I truly getting deep enough?"
It can be something as basic as waiting longer for the fly to sink or maybe even going to a slightly larger size of shot, but don't get into the mindset of always quickly fishing through areas without exploring deeper—even if the difference in depth is relatively minimal. If fishing around other anglers, keep an eye on their presentations as well. If it looks like they aren't giving proper attention to the deeper water, you can often go in and score in those same spots that they just beat on minutes earlier.
For tips on fishing deep, visit this older article right here.