I was told by a tech. (I am a LA quilter) it is a skipped stitch when the needle punctures the fabric and does not pick up the bobbin thread from underneath. If your fabric does not show the holes where the needle went through the fabric it is USUALLY the timing or the stitch regulator. If it is the timing, then the missed stitches will USUALLY occur when the machine is traveling left and/or going upwards as you are standing at the needle side of the machine. Notice the use of the word USUALLY.