That may be normal.
When you don't get aroused when getting ready to have sex, does the arousal come later or not? It's normal not to be fully aroused every time you go to have sex. Some times you're just not feeling it. Nerves can also have something to do with it. Arousal for women is both mental and physical.
I would suggest relaxing and trying your best to focus only on you and your partner. Think about things that make you feel sexy. you can even try doing things to get prepared beforehand. Light some candles, wear lingerie, put on soft music, have your boyfriend wear silk boxers, whatever gets you in a sexy mood. Also, tell your boyfriend when you need a bit more foreplay to get yourself going. Using lubricant in the times when things aren't as wet can help, as well.
If you have other problems, you should see your doctor, but I don't think this is anything to worry about. If you said you never feel aroused, that would be a different story.
I hope some of my suggestions help. The main thing you can do is relax and enjoy the moment. But don't get frustrated if sometimes things don't go as smoothly. Frustration is the biggest arousal killer.