Your bobbin case may just be worn out and needs to be replaced. The Janome bobbin cases are about $35.00 and are available in a lot of places. Attached is a link to Sewing parts online that gives some practical information about bobbin cases.
The first thing I would do is replace the bobbin case itself, or get your repair guy to try a new case in it to see if that is the problem.
https://www.sewingpartsonline.com/bl...e-bobbin-case/