• An unevenly wound bobbin can cause odd stitches.bobbins image by Alison Bowden from Fotolia.com
  
  The bobbin on a sewing machine holds the thread that goes on the underside of the stitches. When the machine is in operation, the bobbin spins while the machine's needle lowers. The needle thread catches on a hook on the bobbin's edge, causing a loop to form. The bobbin's thread goes through the loop while the needle pulls back up, forming the standard machine stitch. Problems with the bobbin can cause the machine to work incorrectly.
  
  

The Thread Breaks

• It's frustrating if the thread snaps when you are in the middle of sewing a line of stitches. Dust build up in the case or shuttle can interfere with your machine's sewing and cause thread breakage. Use the brush that came with your machine or a small piece of damp paper towel to remove any excess dust or lint. Another reason the thread may break is if it is not placed in the case properly. Make sure you've positioned the bobbin in its case so that it fits snugly and that you have threaded the bobbin case as the machine's manual directs. The bobbin thread can also break if it is not wound properly, meaning it unwinds unevenly, creating weird tension in the machine. Check your bobbin to make sure it is wound nice and evenly. Remove and discard the thread and rewind the bobbin if it is not.
  
  

Odd Looking Stitches

• A bobbin that is not wound properly can produce irregular stitches. If your machine starts producing stitches of vary lengths or stitches that are not quite right, stop sewing and check the bobbin. If it is not wound evenly, for instance it is loose, remove the thread and rewind it.
  
  

Tension Too Tight

• Before sewing, make sure your bobbin thread is set to the proper tension. Use two colors of thread, one for the bobbin and one for the needle. Stitch a row onto a scrap of the fabric you will use. If you can see the needle thread on the side of the fabric with the bobbin thread, you should make the tension a bit looser. If the tension on your machine is too tight, your stitches will be weaker and more likely to break.
  
  

The Machine Won't Sew

• A dusty bobbin shuttle and case can cause your machine to jam. Clean the bobbin shuttle and case. Also check to make sure no broken pieces of thread of have gotten trapped in the bobbin assembly.
  
  

Thread Bunches

• Always make sure you have the loose end of the bobbin thread pulled away from the fabric before you start sewing. Hold on to the thread tails when you start stitching so that they do not get pulled under the fabric.