Or just not compatible. If you put a man and a woman into the same room and comeback a few hours later, they won't be mates for life and similar sort of thing applies to many cichlids (including angels). Putting a male and a female into the tank will not result in fry unless they like each other quite a bit, and even if they don't like each other and still pair up, they might just make very bad parents because they are not compatible (for example, might eat eggs, only one might do all the work, might eat fry, etc.)
Also, how big are they? Kids can't have kids, so maybe they just need to grow a bit first.
How big is the tank that you have them in and what other stock is in there? Maybe it will take a few more…