Have you tried prewound bobbins and does that make any difference? Is it sometimes better and sometimes worse? One of the last options we consider is it is a bobbin issue itself, not the casing or the thread or the mechanics, but a tiny little rough edge or ever so slight bend..
The Bernina I have has a proprietary bobbin and that isn't an option -- but maybe your technician already tried/can wind a bobbin for you to try.
My friend's long arm was terrible about bobbins, we tried everything we could and then finally we started buying them via Superior Threads and life immediately got a lot better.
That same friend had a reasonably nice Brother and then got a nicer Brother embroidery machine and while they looked and fit fine to the naked eye, the bobbins were just so slightly different! Finally, she got rid of every bobbin she had and bought new ones, putting a permanent marker line on the embroidery bobbins. Again, life got better
