Troubleshooting Questions to Answer:

Is the THREAD:

1) traveling thru the appropriate holes at the right time?
2) traveling thru the arm hole?
3) wrapped around the tension knob and guide properly?
4) cutting a groove in the tension discs?

Is the NEEDLE:

1) positioned properly?
2) bent or dull?
3) the proper size for the fabric and thread weight?

Is the NEEDLE BAR:

1) lubricated enough?
2) cap cracked? (Barudan)

Is the BOBBIN:

1) threaded properly?
2) turning in the correct direction? 
3) tension set appropriately?
4) clean?
5) lubricated?

Is the PRESSER FOOT:

1) bent?

Are there BURRS:

1) on the hook?
2) throatplate?
3) thread guides?

Is there LINT or Debris: 

1) on the tension discs, keeping them from turning?
2) in the bobbin case?
3) around any moving parts?
4) in the box housing the circuit boards?
5) in the vents?
6) absolutely anywhere?

Have you OILED:

1) lately?
2) the bobbin case?
3) the needle bars? 
4) the areas recommended in your manual?

Do you NEED TO:

1) replace the needle?
2) replace the O-ring?
3) adjust the tension properly for the fabric and thread weight?
4) use the correct backing and/or topping?
5) rehoop or frame properly?
6) start from the beginning, and recheck function settings?