One thing I do when I start feeling that way is to put it down for at least a week and work on something else — either writing a different story, reading something written by someone else or even just taking a break from writing & reading all together. But I always give myself a date that I will jump back in.
There’s been a few stories that I’ve (partially) written and feel like they just aren’t working. Even when I come back to them, I still don’t feel that spark I had when I first came up with the idea. When that happens, I often cannibalize pieces of it into new, different stories.