If the daycare man says "They prefer to play with other pokemon more than each other", then they will never breed.
However, if he says anything else, even "they don't like each other" then they WILL breed eventually, it just takes longer.
Get on your bike and ride around a bit. If you're on D/P/Pt then you can ride up and down the long stretch from north-south of the town. When the daycare man is facing the road is when the egg has been laid (or keep refreshing the daycare poketech app to see if an egg appears).