Sometimes you are just not ready to complete it. It hasn't perked enough in your creative unconscious, the story is somehow incomplete.
So you just need to leave it alone and start work on something else.
Then, maybe in a month, two months or even a year, the idea or inspiration will hit you like lightning, and wham bam boom you will complete this play in a whirlwind of creativity and it will be awesome. Like a fine wine, you let it ferment, grow richer, fuller, layered.
You let it cook.
The creative spark is moody, unpredictable and untameable. When it ignites you need to stay with it, let it channel it's magic, write write write, and when it dries up, acknowledge what is, leave it alone, put it under a rock somewhere, and let it cook.
Let it simmer.
Trust it will ripen with age without your attention.
This is one of the hardest things for most writers to do, but from working with a myriad of playwrights, this is such an important skill for every writer to learn!
It is ok to stop work on a project for a while if it isn't happening.
When I work with my Playwriting Mentoring clients, setting the play aside to let it cook is absolutely unfathonable. They are paying me to help them craft their story, and they go bonkers when I suggest that they leave it alone for a bit and then contact me when "lightning strikes."
"Help me make this happen!" they cry,
and I reply, "I am. Just let it cook for a bit."
You see, you can't make a flower blossom.
You can't make gravy before the Turkey is done.
You can't squeeze inspiration out of a blank sheet of paper or blank computer screen.
If the well is dry, don't try to drink. Find another well to hydrate your imagination for a bit while your creative unconscious does its magic.