I had such trouble with this. First I lost the curve path, because it had appeared “behind” me and I couldn’t see it. I changed the point of origin, which helped with that. Then I got some geometry that went through itself, probably because I had two vertices at the apex. I changed the apex from slightly flat to a sharp point, which fixed that problem but made the rest of the window frame look like an earthworm. I manually moved a couple of vertices and decided that for this window, that was that.
This is probably a good method if the original geometry is precise. I have to assume that mine wasn’t, because I can’t think of any other explanation for the weird skewed bulging that happened to it.
The frame is uneven, because the house is old. Case closed. 
