Question

My Riolu is in the Day Care Center and he is with a Ditto. But ive waiting for 3 days for an egg and there hasent been one. Why does it take alot longer for some pokemon then others?

Answer 1

As DD said, Riolu cannot breed. When you put pokemon in daycare you should talk to the man outside. If he says "They prefer to play with other Pokemon more than each other" then they will never mate.

Anything else he says means they will mate, even "They don't like each other". It will just take longer.

Answer 2

Riolu is one of the baby Pokémons, so it CANNOT breed

Answer 3

Try to breed lucario instead it might work better.