Joe,
I would guess the reason some people have issues with this is that when they put the bobbin case in it 'feels' like it's right so they then go on to sew. "Feeling" right doesn't mean it is right. The case stays in place so people think it's right. I always give my case a little circular twist after it's in. If if moves circular then it's not seated properly. People may think "Okay, it's in correct. It turns." BUT - the case shouldn't turn only the bobbin inside turns.
So...... once you think the case is in correctly - give it a circular twist - if it rotates it's not seated correctly.