Sometimes the bugs that you see are not actually the same bugs that the fish are eating. Even during fairly substantial hatches some fish may key on other insects that are out in far smaller numbers. As you can imagine, this can be frustrating. A friend and I saw this happen recently and were perplexed for a time. Luckily we figured it out and in the end did quite well.
Maybe the best way to really see what the fish are eating is to get in their same current lane, downstream of where they are feeding so as not to spook them, and get right down near the water to actually see what's drifting. If the bugs are still in the emerger stage they can be harder to see so the help of a seine may be required. Sometimes the bugs you're seeing in the air or near the banks are not what you find is actually most prevalent in the feeding lanes or sometimes the fish are simply selectively eliminating the more numerous bugs in favor of something they think is particularly tasty or nutritious.
On the fishing trip I mentioned we found a few baetis drifting in the middle of lots of midges. On close inspection we could see that there was probably 1 drifting baetis for every 100 midges but the fish were only eating the baetis duns. Changing to a size 18 parachute adams did the trick. If you find yourself in a hatch but seem to be getting lots of refusals it's good to try this to see if this might be happening to you. Hope it works for you.