I have also found some plastic bobbins are just bad, irregularities from the process. I also change the needle in these cases in case it got nicked or other issues, and yes, sometimes brand new needles can be a problem too. I became a fan of prewound bobbins over the years but some of those (especially the cardboard ones) also fail too.
Is always so frustrating. I hate it when everything is working just fine and then it isn't.. Usually the old saying was "problem on top/thread on bottom". So yeah, go back through the entire threading process again. Clean the bobbin casing area very well. You mentioned it was a brother and had a thread cutter so it is a reasonably high end machine. It was the bobbin casing unit that started going bad on my first Brother. I could hear the bobbin clunking around periodically it would jump up and hit the top plate. In my case, it wasn't worth repairing. One thing I can say about the bottom of the line Brother I'm using now while my other two machines are down, is that of all things it is very well designed to where you really have to work hard to get the bobbin in the wrong way.