The weight of the fabric can cause the stitches to be too close like that. Put you foot down, loosen the tension and try it to see if that helps. Remember to make yourself a note to change it back after your heavy weight fabric project! A walking foot helps to guide the fabric in a heavy weight or say a plaid fabric that needs to match up. It feed the fabric both from the bottom and the top evenly. Make sure you are using a sharp needle the right size for your thread too. Sometimes that can cause a shredding problem with such a closely woven fabric.