Objective: Identify why a stitch is failing and correct it so your test seam is clean enough to sell.
Most sewing faults are not machine defects — they are threading, needle, or tension issues, and the fabric tells you which. First, always check the UNDERSIDE of your stitching, because the problem usually shows on the opposite side from where it starts. A nest of loose loops on the bottom means the TOP thread is wrong: it came unseated from the tension discs, so re-thread with the presser foot UP (the discs only open to accept thread when the foot is raised). Loops on the top side instead point to a bobbin problem — a bobbin wound unevenly or dropped into its case the wrong direction. On the Janome MC6650 the bobbin should feed so the thread pulls to form a '7' shape as you seat it. Skipped stitches (gaps in the line) almost always mean a dull, bent, or wrong-size needle, or a needle not pushed fully up into the clamp; swap to a fresh needle matched to your fabric (universal 80/12 for cottons, ballpoint for knits). Puckering — fabric gathering along the seam — means tension is too tight or stitch length too long for thin fabric; lower the top tension dial a number or two and shorten the stitch. The thread breaking repeatedly signals the top thread is catching (check for a nick in the needle or wrong threading path) or tension set too high. The golden habit: change ONE variable at a time, then sew a test strip on scrap of the same fabric before touching your real project.
Try it: Take a doubled scrap of the fabric you plan to use for your products. Deliberately create a fault: raise the presser foot, remove the top thread from the tension discs, re-thread with the foot DOWN (so it misses the discs), and sew 10 cm. Flip it over — you should see a messy nest of loops underneath. Now fix it: raise the foot, re-thread correctly through the tension discs and take-up lever, lower the foot, and sew another 10 cm. Compare the two seams and note how the underside changed.