Needle in Backwards
Machine out of time
Needle bent, burred or dull
Lint in upper or lower handling mechanism
Damaged hook
Check spring
Poor thread
Tension too tight
Burrs

Top thread lays flat

Upper tension dial too tight
Lint in upper discs
Thread crossed in thread guide
Not threaded correctly
Lower tension too loose
Lint in bobbin tension
Spool thread crossed
Thread fibers mot the same

Lower thread lays flat

Upper tension too loose
Lint in upper tension disks
Bobbin case too tight
Lint in bobbin area
Poorly wound bobbin
Incorrectly threaded machine
Threads not the same fiber

Incorrectly threaded
Lint in tension disk
Check spring not right
Lint in lower race
Upper tension setting too low
Minor burr on hook

Needle in backwards
Upper tension too tight
Debris in tension
Incorrect needle
Lint pack in lower race
Thread too thick
Needle too thin
Poor thread or old
Burrs on thread handling mechanism
Spool in holder backwards

Needle too low in clamp
Needle bar out of time
Hook out of time
Hook and needle bar out of time
Incorrect type needle
Incorrect needle size
Needle struck pin or foreign object

Incorrect needle
Poor quality thread
Lower and upper thread different weight
Lower and upper thread different fibers
Burrs on fabric handling mechanism
Presser foot pressure incorrect
Bobbin too full
Debris in thread handling mechanism