How large are your rat racks and how long ago did you put them together?
They actually need enough room to be comfortable to breed. If they feel the area is too small, they won't breed or they will breed and end up reabsorbing their babies...
If you just put them together a few days ago, I'd give them a few more days to become friends first. They don't breed instantly like snakes, they're social animals and need to get to know one another.
The other alternative is the females aren't in heat yet, so they're rejecting the male for now. Once theyre in heat, then they will breed.
But it's true that you often don't see them breeding actively- they do it in short spurts that just last a few seconds and then they're done lol. So unless you're watching them constantly, it's easy to miss.