My Kenmore sewing machine Bobbin Holder won't turn... keeps getting stuck?

I have about a 1980 Kenmore sewing machine. The bobbin holder is a front-loaded, not top loaded. When I place the bobbin on the holder, and try to start sewing, the needle (without any thread on the bobbin or through the needle) gets stuck all the way down, like it is trying to push the bobbin holder out. I have removed the two arms, to pull out the bobbin mechanism, put the mechanism back in, locked the arms, and the whole think works okay. Then when I put the bobbin holder in, it gets stuck, and the whole process starts over.... help!!!!!!! I have lost my instruction manual, so you are my only hope!!!