When you trim them down, make sure that the seam is exactly in the corners of the patch, not just that the HST is the correct size. Also, make sure that the seams are all pressed the same way (i.e. all to the dark). When you match the first two hst, nest the seams to make sure that the first half of the unit is correct. If the seams from the 2 patches don't match up at this point, your pinwheel won't either. Press the seams from the 2 half units in opposite directions. when you pin the pinwheel together, use a pin straight up and down to match the points from the 2 half units. When you pin the units together to sew, make sure you aren't moving the fabric as you pin. Might be a good time to try a tiny drop of glue.