Under thread should be drawn out

Excess oil

Select the correct needle

Clean the feed dog

Sharp needle

Blunt needle

Select the correct thread

Needle too small

Take up spring broken

Blunt needle

Excess oil on shuttle

Bobbin too full

Set the presser foot properly

Take - up spring weak

Lower tension too tight

Feeddog dirty

Needle bent

Poor thread

Dirt in the bobbin case

Feeddog dirty or wornout