10 Signs Your Sewing Machine Needs Maintenance: 

1. Unusual Noises: Loud clunking, grinding, or squeaking usually indicate a lack of lubrication, tangled threads, or misaligned gears.
2. Bird's Nesting: Dense thread tangles under your fabric or inside the bobbin case mean something is wrong with your top thread tension or thread path. 
3. Skipped Stitches: If your machine misses stitches, it usually means your needle is dull/bent, the needle is improperly inserted, or the machine's timing is off.
4. Stiff Hand wheel: If the wheel you turn to manually raise/lower the needle feels heavy or difficult to rotate, dirt or gummed-up old oil is likely jamming the internal bearings.
5. Thread Breaking: Frequent upper or lower thread snapping can be caused by burrs on the metal (which need to be buffed) or incorrect tension setting.
6. Poor Fabric Feeding: If the feed dogs fail to pull your fabric through smoothly, lint buildup beneath the stitch plate may be preventing them from rising and moving properly.
7. Needle Breakage: If your needle snaps frequently—even when you are using the correct size for your fabric—your timing might be off, causing the needle to strike the bobbin case or stitch plate.
8. Uneven or Loose Stitches: If adjusting the tension dials doesn't fix looping on the top or bottom of your fabric, it could mean a piece of debris is trapped in the tension discs.
9. Fabric Puckering or Bunching: Persistent gathering or pulling during straight stitches frequently signals an improperly threaded machine or dull needles. 
10. Sluggish Operation: If your machine runs slowly even with the foot pedal pressed, or if it suddenly stops, internal lint or jammed debris may be obstructing the motor.