The finger of the bobbin case holder is probably not in the little groove that is in the needle plate. Every time you take off or loosen the needle plate, you have to make sure the finger is in the slot under the needle plate. Without doing that, the bobbin case won't be securely in and will fall out or just spin. It is because the 301 is a rotary bobbin, not an oscillating one. I have serviced many 301s, and this is the normal culprit.