kay, normal is a very, very wide spectrum and it is more important that you know what is normal for you.

I would first ask you if you can cum easily when you masturbate. Then, think about cumming with him, is it really harder? Or could it be that your Dominant is just not willing to do what is necessary for you to be able to cum. I hate to say this but it is sometimes easy for Dominants to get selfish about their subs pleasure. Don't think you are failing because you don't have a hair trigger--that may just not be the way you're wired.

That being said, I would seek advice from a health professional. Have a complete exam and tell them the efforts it takes to come. They will be able to better tell you if it is really dysfunction of your girlie parts.