How do I remove bobbin case from shuttle?

I have a New Home 657A sewing machine. I was finishing maintenance (oiling of machine; cleaning of shuttle race and under the feed dog (pp31 and 33 of instructions)), when I reassembled the shuttle, retaining ring and bobbin case ready for storage. I tested the running of the machine one final time (having done it prior to taking the shuttle race apart), but it wouldn't run. I then found I couldn't remove the bobbin case, but was able to (awkwardly) remove the case in the shuttle from behind the retaining ring by tilting the retaining ring. With these parts removed, the machine again runs - BUT I cannot remove the bobbin case from the shuttle. Perhaps it's because I didn't put a bobbin in too (since I was going to store the machine)? How do I remove it. And any ideas why the machine didn't run when I'd put the pieces back - everything seemed to fit OK.