are you using the correct needle? you need a knit/stretch needle. loosen the top tension just a little and lengthen your stitch length. what kind of embroidery machine do you have? if its a viking...make sure you use the sewing advisor and put the fabric setting on (heavy knit) that will ease the pressure on the foot so you get less 'drag'.
good luck...try a practice (sandwich) until you get your settings adjusted. but changing the needle (and cleaning the machine if it's been awhile) may fix the problem