She just may not be in heat. Give her a few days, they go into eustrus every 5 days. Wild female rats are often the ones to initiate sexual encounters, vibrating their ears, squeaking and initiaiting games of Catch me!, and actively go after the male rats.

Virgin girls are often unaware of what to do, so they tend to be the loudest in my experience, but he gets the job done. Those girls that have been around so to speak know what's up, and get excited when introduced to a male.