Marion,
First, check your machine settings to make sure the bobbin sensor function is "on".
Did your machine have a round/brown magnet in the bobbin case originally? Did it get lost with cleanings? It is easy to mis place.
It looks like your machine takes item # SA156 for bobbins. These are 7/16" in height. What brand of pre-wounds are you using? Does the same issue happen with them? in the embroidery mode? I use fil-tec bobbins. Are you using the pre-wounds with the card board core? You need the plastic ones.
Does this problem happen with pre-wounds also?
Check to make sure you have the right bobbin case in the machine. There are two that comes with the machine. One is for bobbin work.
Next, Does your machine have all the updates done? What version is the machine on now?
Is this your machine?
https://support.brother.com/g/b/manu...inovvm6200deas