I've never had this happen mid-chapter, because I storyboard/plan an entire chapter in one go - but I have had it happen between chapters, and I certainly had it happen before I got started actually drawing Grassblades.
In the first instance - where it happens between chapters, where it's certain scenes that aren't cooperating - I usually leave it alone for a little while, and get busy doing something else. The way my brain is wired, it seems like I have an easier time solving plot-issues if I'm caught up in drawing/writing something else - it takes a bit of the pressure off, and lets my brain kind of run free a bit, which is the best way to get over obstacles that seem immovable.
Another way - if the first doesn't work - is to do sketches/storyboards/written notes on various alternative ways a chapter or scene could end. Just play with the possibilities without committing to any one of them; you're just exploring, and you're not bound by any decisions yet.
Sometimes, it really IS a matter of smacking your forehead against that creative wall until it crumbles.
The important thing is to keep at it - leaving a project alone for a while is a helpful method, but you have to come back to it. Just wandering off will not solve your problem; it'll just leave you with an abandoned project that will never really see the light of day - and we don't want that. Finishing things is important. So take a day off, watch a movie, play a videogame, draw something else - whatever helps you relax - but come back to it. Set yourself a deadline for checking back in; it doesn't have to be a deadline for starting work on it again, but just a little mental reminder that keeps you coming back to the project.
Mental blocks on projects can really suck - Grassblades sat around for literally years before I got started drawing it, because I just COULD NOT figure out an ending, or an antagonist. But I kept picking at it every now and then, and last year, like a bolt from the blue sky, I came up with something.