The problem lies with your Needle.

Did you change the needle
If you did did you put the flat part of the needle against the post that holds the needle clamp. If have a manual consult it for right needle placement. If you didn't change the needle then---

Is the needle bent
It may not look like it but change the needle anyway. Sometimes it is just a small bend but it affects the way the machine sews. When I sew I change my needle often

Is your needle too fine for the thread being used
EG--a size 11 needle would not work with buttonhole thread.

The needle would be the 1st thing I'd check as well. There's a groove down the length of the needle & that's the side where the thread goes in. You can usually feel it w/ your fingernail.

& have you checked the manual for troubleshooting tips yet?

HTH