Edit note: Glenn is 100% correct about the thread going under the lower curved portion of the spring also.
I'm lazy..... I do not like to try to "guess" what is wrong with components that have multiple elements, so.....
1. Put the bobbin in ONLY and pull the thread gently and slowly.
----- If you feel anything but smooth consistent release of the thread clean the divots and pin ends
2. With the bobbin in place, thread it around the rear bar and do the slow gentle pull
----- If you feel anything but smooth consistent release of the thread wrap 0000 steel wool around the bar and clean it.
3. With the bobbin in place and thread around the bar, put the thread through the slot in the front and gentle/slow pull
----- If not smooth consistent release of the thread VERY gently work toothpaste on dental floss through the slot and clean VERY well afterwards.
4. Check the point of the shuttle for sharpness. Does not need to be like a pin point but sharper is better
5. Check the needle height. (you can remove the slide plates and shuttle and very slowly operate the machine through one cycle and watch the loop size and location when formed.
6. Lastly, check the shuttle holder position. They are usually adjustable to accommodate for top edge of the shuttle wear, but are almost never adjusted.