elnan

When you cleaned and checked everything, did you remove the throat plate to clean around the feed dogs, etc? It is when replacing the throat plate that you seat the position finger of the bobbin case in the notch. Once those two parts are in place, the piece with the finger does not come out or rotate, no way for it to fall out. Set the bobbin and case aside, remove the throat plate and replace it and the position finger at the same time, then thread and insert the bobbin and case as MFord has said.

I find this easier to work on if the machine is standing on it's end and bolstered by something to keep it from falling over.