I'd check off contact and make sure you have about the thickness of a nickle between the shirt and the screen. You can table up and press the mesh to make sure there is clearance.
I would then more to squeegee pressure. Are you clearing the screen fully? You might have to double stroke the print(i know people are cringing right now, but he can get the job out the door and then work on fine tuning everything).
If you have too much pressure you are driving the ink into the shirt and picking the fibers up after it prints.
There are tons more things, but these are usually the ones that get us.