Is your presser foot flat? If it is tipping as you sew, you will get skipped stitches. This happens when going over thick seam allowances in jeans, for example. If this is the cause of the skipped stitches, the solution is to place a shim under whichever part of the presser foot that is tilting down -- usually the back of the foot, but could be the right side also. I am wondering if the seam allowance in conjunction with your distance from the seam is causing your presser foot to tilt to one side (higher on one side or the other). The shim can be a small piece of plastic or cardboard.