Go back and measure each seam, to see what's causing the problem. If all else fails, I'd just make the pinwheel section a tad bigger, then trim it down to size.
Another reason for the problem, may be your HST seams. Some patterns assume you'll use a
scant 1/4" there. I never did like the scant 1/4" practice, as IMO the bias seams need more stability, not less. However, many people seem to think scant seams are the way to go.
