if they dont succeed the firs time, dont worry, once they start spawning you will have more spawns, and everlastace is right, sometimes it takes a few attempts, or sometimes they never get it and you have to raise them artificially. it depends on the pair, you just have to give them some time, and if you havent already, try to mimic their natural habitat as much as possible.
i was reading in a magazine about how to get better results from angelfish spawns, and what they talked about was adding a lot of driftwood and plants, have them alone in a tank, and cover the bottom, back, and the sides of the tank to make them feel less stressed (just make sure they have some room to swim). the reason for adding lots of drift wood is because that is where they would hide in their natural habitat, and their body shape is great for swimming through the wood and getting away from predators, so adding something that is a familiar safe haven to them could really benefit. personally i havent tried this method, but it would be worth a try.