For me, it sometimes depends on why it's not working, and if I know what the reason is. Some things I've tried that have worked:

-Work on another part of the outline
-Work on another piece of writing entirely
-Do some chores or take a long walk
-If it's something where I'm having trouble getting a sense of the action, blocking it out
-Ask myself if it's my subconscious trying to tell me the scene doesn't need to be there
-Try writing it from another angle
-Sleep on it