I've written songs for the past twenty-four years, and there are a handful that I have yet to finish. I personally refuse to force a song, because chances are good that a forced song won't be (good).
I would recommend leaving it alone for a while, rather a couple of days or longer if needed, then allow it to call out to you. You'll feel it when it's ready, and you'll know it when the time is right.
Many times I've worked out the tune to a song on my guitar, get it all written out on paper, and then when I play and sing it, I do it differently. The most important thing is that it flows.