Some lakes have a lot more action for fishing dry flies than others, but generally it's timing. As other have said, most fish will feed below the surface.

However, I like to think of if this way when it comes to fishing dry flies on a lake. It's not if they'll take them, just when. It's all a timing game. Some lakes are more predictable in timing the hatch than others. Also the time of year plays an important factor when the hatch comes off.

Also watch the swallows. If there are a lot of swallows flying around the lake, then you know there are a lot of bugs hatching off the surface. When they go away, the hatch is done for now. But, the swallows will be back, along with the fish, at some point.

Last week Brad, Brian and I fished a west side lake that I never fished before. It really surprised me the number of swallows working the lake ALL day. There were a lot of bugs hatching. These swallows were the fattest swallows I've seen too, porky little guys. The fish were off and on taking dries through out the day. So, you just have to be ready for them.