Numa,
Do the feeddogs move freely with out the plate, then not when you put it back? Does the needle go straightly through the needle plate?
Remove the bobbin and bobbin case.
Turn the hand whell until the needle is at its highest point.
Replace the bobbin and bobbin case firmly.
See if the top thread will loop the bobbin thread while you hold both thread ends.
If the feeddogs are not fouling on the stitchplate/needle plate cover, I have no other idea.
Aardie.