I've been dating my boyfriend for two years now and we have amazing sex. The only problem is, he can't make me cum. But.. I know I'm capable because I've done it before with my ex who I wasn't even attracted to... It just doesn't make sense. What is wrong with me?
How does your BF's cock compare to that of your ex and how does his "technique" compare. If he's thrusting deeper or shallower, at a different angle, or if his cock is just shaped differently (more curved, less curved, longer, shorter) such that it doesn't touch you in the same places inside, well, any or all of those factors can affect the type and amount of stimulation that you receive. Then, there's the type and duration of foreplay, etc. The list of possible factors is lengthy and complex.
You should masturbate more. Masturbation will teach you what you need to know about how you want him to perform. Then, you can teach him.
Absolutely nothing is wrong with you it just comes down to technique. Your ex obviously did it in a way that worked for you, and now you're with someone else, so you need to figure out together what works for you. Communicate, communicate, communicate.
I agree with all the replies above. I want to add that there are some guys who just don't care whether you orgasm or not. They only care about their own orgasm. On the other hand, others have the will to make you cum but they don't have the skills and techniques.
AussieDude is completely right, technique, communication, and trust in your partner during, before, and after sex is essential, and can be the difference between having an orgasm and not being able to.