I found this a rather enlightening article :
Particularly this bit :
Getting stuck is a big part of creative work, and it’s really important to be good at getting unstuck. There are two main reasons why creative people get stuck on a piece of work: The first is, you don’t actually have an idea. You may have requirements, and you may have tools. But you don’t actually have an idea that’s going to carry the day, and you’re going to be stuck until you get a solid idea.
The second reason creative people get stuck is that, while they have the idea, executing the idea takes a lot of work, and not all of that work is fun, and basically you don’t want to do the work, because having the idea in the first place was the fun part. The problem is, you don’t get to say “check mate in four.” You actually have to finish the project. So you get mystically “stuck” after the brilliant sketch is done.