I'm going to assume the Nursery response was "They don't seem to like each other very much". This means that the Pokemon you have deposited are compatible, but they have the lowest chance of generating an egg each egg cycle -- it comes to a measly 20% chance. You will eventually receive an egg from the pair, but it may take several cycles.
If the response was "They prefer to play with other Pokemon...", then the pair you have deposited are not compatible and can't breed. In your situation, that would mean that either the game is glitching out, or one of the Pokemon is legitimate (created via hacking) and the creator did something wrong. If either of those Pokemon came from Wonder Trade, for example, there is the possibility that the Pokemon was not captured, but created via hacking methods. If you are sure you obtained them yourself, I would suggest taking them both out then putting them back in and see if that changes anything.
