Maukka

Sometimes check spring does not move freely, you can try to reshape it with small pliers. That ring at the end of spring behind knurled knob should move freely. I have added an bigger washer to check spring assembly to make it work better. These are wonderful machines which have to be fine tuned to work perfectly. Decades of use by operators who don't understand machines makes no good.

Looks like upper thread is wrong way. It should be behind that little finger at check spring assembly.

Have you checked the needlebar height is right? And play with check spring tightness and position. If it makes any difference.