How long since they have spawned, and what is the temp of the water?
Males will move the eggs around in the nest, oftentimes I will start seeing less and less eggs in the nest because the male will take one out, clean it and stick it up higher into the nest. One time I didn't see any eggs, but ended up with a few dozen fry :)
Is the male still tending the nest? Is he staying below it, building more bubbles or just watching it intently? If so, then he still has eggs up in there. The fry are also very very tiny and it is easy to not see the tails right away.. give it a couple days before giving up on him.. look for tiny tiny specks in the water after a while. Keep in mind, the lower the temp, the longer it will take to hatch.