It has happened to me so many times that I gave up writing full length plays and went to ten minute plays instead.
Here are a few observations by others:
Romulus Linney: Playwrights who really work at this (writing) often have works around in many states of completion, in various stages. So when you're working on one and you hit a stone wall or need some distance, you put it aside and you say, "All right, what else is around here?" And you pickup something else. It's a never-ending process.
Michael Weller: Half of getting through a first draft is just being too stupid to know how bad it is and to just keep going and going and push through to the end. Then you go back and say, "Okay, how can I persuade myself that this thing really happened.?"
For me it's been one of two things: Either a problem of not having something that excites me at the end of the play. Each play needs a magic moment. The single beat in the play that has the audience so enthralled that they stop breathing. Putting it differently, if I'm not excited about getting to my destination I can't get excited about the trip. My best work is when I have something that really excites me at the end of the play. It may be a great climax or a wonderful denouement or, in a recent play, a single pistol shot that actually happens in the dark after the play has ended. Each of these excited me enough to make me slog through all the dialog to get there and to set up the magic moment.
Or it might be what I am writing: I have three kinds of plays: Plays I like, plays others like and plays no one likes. So far I don't have any that I like and others also like - well that may be a bit of an exaggeration but the plays that mean the most to me are the ones that others don't like. I write them because there is something that I need to say but I haven't figured out what it is yet. When I am writing them, I find out that what I am writing is not what I want to say so I put them aside and start on another play. The unfinished ones are not failures but things I've learned about what I need to tell people. As I work on this unknown message, my plays are getting longer and more complex so it is an overall gain.
I also use the trick of putting the play away for some time and thinking about the play and what I want to say with it. I usually wake up with some ideas that gets me back to the play again.
Don't give up.
Doug