When you have a coarse weave such as an acrylic knit hat or pique knit polo shirt, you can get the embroidery to come out well with digitizing if you are a jedi master of embroidery, but a simple “trick” for mere mortals is to use Solvy on the surface.
Solvy is a plastic film manufactured for the embroidery industry. It is placed on top of any coarse substrate with an open weave, for example an acrylic knit hat or pique polo shirt. It makes the stitching sit up on the material and you get more complete fills and more detail on lettering. You place backing on the piece as you normally would and sew it normally, and then afterwards tear the Solvy away by hand. If small bits of Solvy remain, you can remove them easily with a nylon brush.