This may not work for everyone, but it has helped me and so there's at least the possibility that will help you, too.
I have learned to never focus solely on one project. If I ever only have one thing I'm working on, I'll invariably write myself into a writer's block. It's such a strange thing. It's like navigating an unfamiliar city. You might have a map. You might have traced the route you need to take. But at some point, you're going to stop on a street corner, stare at the map in front of you, and see absolutely no relation to the physical place you now find yourself standing. Sometimes the map can help you out and sometimes you just have to ask a local, "where am I and how did I get here?"
In that metaphor, the local was played by Second Writing Project.
It helps if the second writing project is a completely unrelated story with completely unrelated characters in a completely unrelated genre. Because writer's block seems to happen (for me, at least) when I get so wrapped up in the story that I can't see the big picture. I don't see that this little detail that I'm stuck on doesn't really matter. Sometimes, of course, the words just don't come to me. I know what I'm supposed to write, but it's just not there. The screen is blank because my mind is blank. But the mind isn't so simple that it can only focus on one thing. You can have all sorts of simultaneous thoughts bubbling to the surface at any one moment. So just because this project isn't one of those ideas doesn't mean you don't have other legitimately good ideas going on.
Close that document and open another. Work on something else. Write the next paragraph of that other story you're working on. Jot down that scene you've been playing over in your mind all day. As you play through those scenes, more times than not, you'll inadvertently crack the code of the other thing you're writing.
That's how I do it. That's the only way I know how to do it, really. At any one time, I'll have two or three things I'm working on, writing, dissecting, outlining, or drafting. Don't limit yourself to one project at a time. Your brain is a pretty amazing thing.