I can think of a few different issues it may be.
1) She may be a hot female which means she was incubated at 90-92 degrees and if she was then she will be agresive and not breed very easily if at all.
2) It may not be breeding season, she might not be ovulating and don't want him bugging her.
3) She might not like him, I have read that if she don't think he is attractive ( have the right build,too small) she will fight him off.

If it is breeding season one thing you can try is just to leave him in there for a couple weeks and let her get used to him she might be more receptive. Now that does depend on how bad she bites him if you are seeing bad bite marks get him out. Hope this helps. GL