Is the selector engaged can you tell? If it feels loose, pressing it gently may fix it,if not, unplugginh the machine and then opening it to get a look may reveal a loose spring, broken plastic, dried grease, etc. If none of this solves it, let me know.
Hi, pre threaded bobbins are a universal fit provide you make sure that your bobbin, whether it be plastic of metal has concave or flat sides This is most important. Just match
them to your bobbin and you will be OK. .
your machine will have a flip type opening at the very front of your machine above the area where the needles are attached to the arm that moves up and down ( it usually has the light globe installed in there ) after opening that door you will see where the dog feed is attached to it should also have a spring that helps the dog feed return to the up position when not in use , make sure it is not stuck in any way and some lubrication oil should be used to prevent it from sticking , while using the dog feed release button push the spring in a up and down motion to help it to dislodge