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Take a break. Go out, do something else. Find a magazine or a book, talk to some people, go sit on the curb and watch the cars. Do something that ISN'T the projects you're stuck on. It doesn't have to be for a long time, but enough time that you can clear your head and relax doing something that ISN'T art.

It may seem counter-productive. When it gets right down to it, doing this is seen as counter-productive by a lot of people. But you need to look at your creative self as a rechargeable battery as opposed to an unending force that can be tapped at will. Sometimes, you just get drained, and you need to give yourself time to recharge.

When I'm stuck on a scene or I've stumbled across a character that I don't know how to write into a situation anymore, I take a book and a thermos of tea to a local park. If I'm 100% drained of absolutely everything, I take a kyak trip into the middle of nowhere and spend a few days fishing and cooking over a fire and enjoying being completely disconnected from my life in the city. In the winter, I'll hijack my uncle's lakeside cabin for a few days, unplug the TV, disengage the WiFi, and leave the phone off the hook, because disconnection is how I recharge.

I encounter the same problem, started but never finished.
Sometimes I start 5+ pictures that never get finished but my best drawings usually come out of that.
I've been drawing my oc's and practicing poses so I havent had this problem recently.

Sometimes just doing avi art may help though.