Could be several reasons. One would be a finish on the fabric that is resisting stitches and another reason could be that the fabric is off grain and the needle is not lining up with the bobbin thread to make good stitches. I have experienced both of these problems. You can try these things: try changing to an embroidery needle or micro-needle or titanium needle, sew over tearaway stabilizer and remove after stitching, and try another type of thread. Hope you are able to resolve the problem, good luck.