One of my drawing teachers once told me that the best thing you can do in these cases is to withdraw for a while from what you are creating, have a coffee with a friend, walk around, read a book and let your mind not be saturated, then you come back and see things that you couldn't see before because of the blockage

This is my favourite advice and one I practice often. You can't force creativity and sometimes your mind just needs that rest. It might need a day's rest or even a month's rest, but when it's ready the floodgates will open.

:)