I am using cotton-covered polyester thread to sew the shirts, and the buttonholes in this thread were not the best. The stitching on the right-hand side of the buttonhole was a lot looser than the left side and sometimes the center space was way too small.
When I looked at a commercially-made shirt, it looked like they were using a much thinner thread. I decided to try some Maxi-Lock thread in both the bobbin and top threading but using the same needle and buttonhole foot. Here’s the buttonhole produced: