Every girl has a different technique, but I've never had a problem. Sometimes it does take a ridiculously long time but that's because the girl doesn't know what they are doing and isn't applying enough pressure, doesn't know how to work the hand and mouth at the same time, etc.
I find it difficult usually but rarely i can. Psychologically I think it has a lot to do with your experiences when you first started having sex and the bj's i had back then werent good. Physcially i like being in control a bit too much for it to happen...but not always.