I have a tin lizzie and haven't had this problem. I will say that my dealer told me to insert the bobbin casing while holding that little lever open. I had the bobbin case fall out once or twice. Since then I've learned that you are not supposed to hold the lever open. Position the case and press until it clicks in place.
There is a board dedicated to Tin Lizzies on Yahoo. They could probably help you more there. Just google "tin lizzie yahoo group".
Hope you still visit here!