I wish I knew the answer here. All my girlfriends are reluctant to make love as often as their men would like. Why? Maybe women are geared to have sex to make babies and if we aren't interested in making babies we aren't interested in sex. Don't take it personally, lots of my lady friends say the same thing. On the other hand, sometimes asking isn't the correct approach, ahem, I said the right APPROACH...in other words, don't ask, right. But be sure you remember that "No" means "No" and stop if your approach is not welcome. And remember that females are romantic creatures, so some romance ups the odds...are you listening Jumpin' Jack Flash?
Maybe she doesnt like you asking for it. Maybe you guys arent meant for each other. You never know if she is using that as a means to control you. You might want to talk to her about it before you start stepping out to meet your needs. That would NOT be a good thing.... but people seem to do it more and more often. Open the lines of communication FIRST.
Are you married?? My exwife was the same way!! Dated her for 7 years and married for 13.. I used to get teased because I would complain that she wanted it too much.. Sign that paper and slap a ring on that finger and she quit!! I used to tell her that I wanted to get divorced so we could start doing it again!!! She didn't think I was as amusing as I thought I was!!! Lol!!!