I have struggled with a machine or two that were bobbin ejectors. There may not be a simple answer and I may not have your answer.
Does the bobbin fit the bobbin case with out sticking out just a wee bit? Is it the correct Bobbin? Is it the correct bobbin case? (Likely yours IS correct, but is it a brand new one or original?) is the bobbin case clean of lint, dried up oil etc way inside? I have goten into the habit of taking mine apart routinely and cleaning when I get a machine On my bench. Is the bobbin warped or in some way damaged?does it happen with all your bobbins or just certain ones? Is there a plastic spacer or something foreign in the bobbin case? Can you trade bobbin cases with another like bobbin case and see if it does ejects too? Is there lint in the shuttle or under your throat plate? Does the spring on the bobbin case work? Let everybody know what you do find.
I just had a machine - not a 301 but a 15 type. It came with no bobbin case and I put one I had on hand in and it ejected. I found I needed one with a different slot. This in no way is relevant to your 301 but for someone else it might be helpful.