Sounds like this might be made with a lot of triangles? The first thought that came to mind was the triangles not being cut to correct size - thus finishing out a smaller size than expected.
I feel your pain though. Been there a few times - sometimes because of the foot on my machine and sometimes just because I figured the measurements wrong in my head. I agree with stepping back for a while and coming back to it. It's easier to think of a solution when you're not full-blown frustrated.