Some of the common troubles and their possible cause are listed below. You can take care of most of these by yourself. In case of major troubles the help of a qualified mechanic should be obtained.
Bobbin too full 2. Bobbin set in wrong direction 3. Under thread not drawn up
Both threads not pulled back under the presser foot 5. Machine not properly oiled and cleaned.
Needle bent 2. Needle set with long groove turned to wrong side 3. Needle too high or too low in the needle bar 4. Needle too small 5. Needle threaded from the worng side 6. Excess oil on shuttle.
1. Poor thread 2. Machine incorrectly threaded 3. Needle set on wrong side 4. Needle too fine for thread 5. Needle threaded from the wrong side 6. Upper tension too tight 7. Sharp edges on needle plate hole or shuttle 8. Thread take-up spring broken.
Poor thread 2. Lower tension too tight 3. Bobbin case threaded wrongly 4. Sharp edge on needle plate 5. Bobbin wound too full or uneven 6. Dirt in bobbin case.
One or both tension too tight 2. Stitch too long for material being sewn 3. Blunt needle.
Incorrect size of needle for thread and fabric 2. Needle blunt 3. Pulling of material while stitching 4. Presser foot incorrectly set 5. Crossing a thick seam using a too small needle.
Too little pressure on presser foot 2. Take-up spring weak, broken or missing.
Incorrect presser foot pressure 2. Feed dog dirty or worn out.
Stitch regulator set too close to '0' point 2. Dirt under needle plate near feed dog
incorrect presser foot pressure 4. Bent presser foot.
Lack of oil
2. Thread wound round wheel or treadle bearing
3. Belt too tight
Bobbin winder pressed down
5. Thread jammed in shuttle race
6. Gummed oil or dirt on bearings.