There's a handful of things that might be going on:
1. Your tippet is too heavy for the size of the fly, so it gets pulled under in short order.
2. You're imparting to much action to the fly. Typically, dry flies are cast and let to drift with the current. In the absence of current, the fly is cast and left to sit. Occasional twitches that barely move the fly are usually all of the movement necessary.
3. You're not treating the fly with floatant. There are many sprays, pastes, and such that can be put on dry flies prior to fishing (sometimes even the night before) that will help a dry to float for an even longer time.
Keep in mind that if your fly becomes waterlogged, false casting may be able to dry it out enough to get a few more floats out of it. And FWIW, tiny poppers are dynamite on panfish and float forever without treatment.
Glad to hear of your success.