is your pair a black melano pair? And if so is the female black herself? All black melano females are unfertile. That is why we always pair up the black males with their (fertile) blue sisters. If someone sold you a black melano female to spawn, I would definitely be pretty upset about it if I were you.

your pair did not properly spawn. During the spawning process, the male will have to properly wrap his body around the female so that he can fertilize the eggs as they come out. If the male is not in the proper position, his sperm will be too far from the eggs to reach them and penetrate them... (is this site becoming a XXX site? LOL). The eggs will not be fertile and not hatch. If your female is much too large or much smaller than the male, it could cause such wrapping technical problems.

fungus attacked your eggs. If the pair spawned properly, the eggs should be fertile but may not be able to hatch because a pathogen (fungus) latches onto them and spoils them. It is recommended to always add a few drops of maroxy in the spawning tank for that reason (to prevent fungus). Usually works pretty well.

your male is unfertile: This is rare but does happen on occasion. A male may just be shooting blanks. The only way to know for sure is to try him with different females. If you always have the same result, then the male is at fault. On rare occasions a female (and not a black melano female for that matter) may turn out to be unfertile herself. She lays eggs, but the eggs do not have the ability to produce fry. Again you won't know for sure until you try her with other males.