Bobbin not not threading, what's the problem?

SOURCE: Problem with winding thread onto

first you must make sure the winder shaft is turning by moving it to the right a little and see if you can turn it by hand.
If it is turning ok then there is a rubber ring on the other end of the winder shaft and it should come in contact with the drive which turnes it ,if it is worn it might need a new rubber.

SOURCE: Upper thread wraps around bobbin mechanism, while sewing

IT PROBABLY IS THREADED INCORRECTLY. GO SLOWLY BY THE DIAPHRAGM AND MAKE SURE IT IS THREADED CORRECTLY. ALSO MAKE SURE SPOOL PIN HOLDERS ARE IN PLACE THAT HELPS TO GUIDE THE THREAD CORRECTLY.

SOURCE: Upper thread knots up with bobbin thread when sewing a stich

Thread test
Test your thread quality to start, thread your machine and LIFT the presser foot (this opens the tension disks). Pull your top thread straight back. If you feel no tension no mater how much thread you pull, your thread is good. If that your machine passes that test, then let me know, we can go to step 2. If not, try different spool of thread, same test until you get a smooth pull (tip: always check your thread this way when you thread your machine)
Step 2:
Pull on your threaded bobbin thread while it’s in the machine. Does it pull smooth and even? If so, go to step 3. If not, try a new bobbin (bobbins get bent or distorted if wound too tight).
Clean thoroughly in the hook and bobbin case area and oil 1-2 drop is all.
Thread knotting on the bobbin can be a lot of things but here's some guidance:
Tension:
If the thread is loose on the bottom of the fabric, it's actually the top thread is too loose. Think of 2 little elves playing tug of war in your machine, one on toip and one underneath. If you have loops on bottom, the top needs to pull harder (tighten top tension)
Jamming bobbin case:
If it is damaged from turning out of place once, it could have rough spots on it that makes the thread hang on it, and keeps making it turn over and over. Use a finger nail board (fine sanding) and smoothall rough spots. Then reset the bobbin case taking care to put the notch in the bobbin case in alignment with the proper spot in the machine (basically 5 o’clock) when looking at the round area where the bobbin case goes.

SOURCE: The thread from the needle that goes down to pick up bobbin doesn

make sure your needle's flat bit shows to the back and it is in deep anough. else your timing is out.

SOURCE: Singer Sewing Machine model 9224. Bobbin thread

If you turn the handwheel, can you see a metal ring rotating around the outer edge of the bobbin case? If not rotating, broken gear or timing belt off.
If that ring is turning, remove the needle plate and bobbin case. As this ring now rotates, you will a finger on the ring that will pass behind the needle from right to left and it should catch the thread from the eye of the needle. If this finger, the "hook" passes above the eye, you have a timing problem.

Let me know.