There are many factors into getting good results. Hooping and stabilizers are contributors.
How dense is the embroidery design? Lighter stitch count designs work better.
What stabilizing technique are you using for the shirts?
Fuse a fusible product on the back of the shirt. Hoop a layer of cut away. Smooth the shirt onto the hoop. Add a basting box around the design.
Depending on the design, you may need to float another layer of tear away under the hoop.
Slow the machine speed down for stitching.
Helpful guide.
https://rnkdistributing.com/floriani-product-guide-2/