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RiverRatDC has good suggestions for fine tuning the fit, but it sounds like that may not be the issue here.
The other thing that comes to mind for me is that there are two sizes of corner gears. The large and small gears need to be on alternating corners. In my pictures, the yellow corners are large gears and the blue corners are small gears. See if maybe you have small corners next to each other.
Hope this helps!