Put them alone together and supervise them for a while. Sometimes rats just need to show their pecking order and it takes a little bit of posturing before they know who's the alpha. However, if you get two alphas you might end up with an injured or dead rat which is why it's a good idea to supervise them until they settle down.

Typically that won't happen if the male is much larger than the female, but if they're about the same size the female might not want to be bullied around.

Good luck!