"He can't cum!"
Sometimes when giving a guy head, it will start taking him FOREVER (or so it might seem) to cum, your jaw will start hurting, or your neck, your mouth my get wet or you just start wondering whether you are doing the right thing. Most girls mess up at this point - since they start making strange faces, from which the guy can see that you are getting uncomfortable and that you started thinking too much - instead of enjoying the blow job. And if YOU don't enjoy it - and he sees that (it's not hard not to see), HE will stop enjoying it - he'll feel guilty that he is "forcing you to do something that you don't like" or think that he's torturing you when you're giving him head.
As a result - he'll start thinking - A LOT - and it will take him EVEN LONGER to cum (but he probably won't cum at all) he might even lose his erection. There's a solution to this problem - and every other blow job problem you come across. One way to solve this is by learning how to control your "blowjob energy" and use the right tempo. Suck a bit, rest a bit, suck a bit, rest a bit - you don't have to go straight from start to finish at the same speed. Have FUN, make it more interesting, adapt to the current situation. Sometimes you'll give him a fast blow job, he'll cum in 3-5 minutes, other times - you might blow him for half an hour. Learn time management. Over time and practice, you'll know the guy better.
Never start THINKING while giving a guy head - getting into your brain and moving around thoughts like "why didn't he cum yet?", "Am I doing this right?", "do I look dumb from up there?" will RUIN the blow job! Be there - in the moment 100%, even better 110%! You have to fall into a completely sexual state of FEELINGS and pure ecstatic state of mind from sucking his cock.