actually this may also a tension problem. If you look closly you will see that with all the stitching you can see the top thread /it is pulled to the bottom( this looks like the backing). this means that the bottom tension is too tight therefore pulling the top thread to the bottom. I was taught as a home sewer to test the maching tensions by sewing a straight line on the bias/diaginal. Then gently tub on the corners and see if either thread breaks. If the tension is good then no thread should break. If the top thread breaks then the top tension is too tight , if the bottom thread breaks then the bottom tensions is too tight. I was also taught to try fixing the problem by adjusting the top thread and never mess with the bottom tension.