Sometimes, a man will masturbate a lot. And they do it very hard and fast and as a result, a mouth isn't strong enough to give the lover the appropriate friction needed to go over the edge.
Another poster was making a joke about it above. I tend to refer to it as male dish pan hands.
I had a client several years back that actually had to go to some therapist because he had gotten to the point that the ONLY way that he could climax was to jack himself off while having intercourse (also, he was humongous, which was a negative issue in itself).
I've made the suggestion that a man either stop touching himself for awhile, or start to pay attention and change up his technique. Try to give yourself an orgasm while doing something, anything, differently.
And then keep varying it up.
Most people get into a certain pattern of touching themselves and sometimes, it translates into having problems ejaculating any other fashion. Especially if you haven't had a regular partner, or regular sex, in awhile.
Again, just a guess. Or it's nerves.
There are a lot of men who are just used to having an orgasm through intercourse and that is what does it for them.
The mouth can be too threatening, with the teeth and all, for some men to be that vulnerable to a stranger. Just my thoughts on this situation.
Us ladies do see this sometimes. So it's normal enough.
Good luck. Frankly, I think that it's kindof cute that you started a topic about this issue. Now, you're going to have all of these sex workers wanting to help you out with your situation.
They'll want bragging rights, too, I'm sure!
Men DO seem to like orgasming while thrusting into the mouth of a female (insert joke here).
Best,
Elisabeth