Writing is the thing I'm most experienced at. It's also the thing I find hardest. In fact, in the past few years I've hardly been able to do it at all.
Alan Garner wrote that "There's no such thing as writer's block, only writer's impatience." I half agree with him: trying to write "seriously" when you don't have something genuine to say is asking for trouble. However, it's not the only reason writers get stuck.
Frankly, unless you're supremely well-adjusted or utterly rhino-hided, simple fear can strike you dumb. If the last thing you wrote was rubbish, why should the next one be better? - and if it was halfway decent, what if the next one is disappointing? - and so on, unto paralysis.
I've written one book. Maybe I'm a one-book writer. Feeling bad about this used to stop me doing anything else creative - until I realised that making (other) things is much more fun than Not Writing.