Ok, first things first. How old are the Angels? When did you get them? What size tank are they in? What else is in the tank? And finally, did you remove the slate to another tank or leave them to the parents to raise?
Second thoughts are most Angel parents eat their first few batches of eggs due to immaturity, sometimes they never grow out of this habit thus forcing the fish owner to try to raise the babies on their own if so desired. When this is desired, the eggs are laid on the slate or another easily removable object, then once laid and fertilized, it is removed and placed into a breeding tank. Here, the water is treated with methylene blue or something of the sort to prevent fungus from getting to the eggs, and fixing an airstone to softly brush air over the eggs constantly.... this is usually done by the parents of the eggs by simply fanning them with their fins.