gaigai
Check to see if there are dust bunnies amongst your feed dogs. sometimes the least little bit of lint can cause that problem. I would also check the bobbin case for lint because it makes a big difference. Some fabrics such as flannel, and certain threads can create problems for the feed dogs and for the bobbin area. That could make the feed dogs act up. I have brothers and other brand machines that I use, and some are touchier than others.
good Luck
Suzy