I went through this a LOT with my ex(alas, we never had sex. got caught by my parents, didn't see eachother for a month, and broke up shortly after that), and I know for a fact, it was almost (if not completely) all my fault. I had the foreplay down pretty good (biting, licking, kissing, going down on her, biting her nipples, etc.) but when it came to sex it just didn't happen. She said she "wanted to have sex with me, but couldn't". After trying to alleviate the problem, I turned to outside sources for help.
One of the biggest things was that I was too pushy. If you're EXPECTING to get sex and HAVE to have it (outcome dependent) the girl will not want/be able to have sex with you because she can sense that.
Another thing was I wasn't opening her up with my masculinity i.e being a true man and being confident, dominant, leading, etc. I let her call the shots all too often, and that mixed with me HAVING to have sex with her(obsessing about it is a turn-off to all girls) ended with finger and tongue-only. However, the "just the tip" thing might work, but I never thought to try that, we grinded naked(lol wtf?) so it wouldn't have been hard to try that.
Also, read The Way of the Superior Man by David Deida. Sadly, I read this book after it was too late, but it opened my eyes immensely. Also head Wild Nights by him is amazing, but I haven't read it yet.
Best of luck. Get some!