Bobbin thread not trimming

[Hidden Cottage]

Hi,
I have been running good quality flour sack dish towels with DOW, Redwork designs from Emb. library. Not using backing, but bobbin, almost never trims, causing all kinds of trouble. Had to do 5 sets of 7 each, for a gift shop, sat next to the machine and had to keep watch. Sews anything else just fine. Any suggestions? We're supposed to be able to go with backing.
It's good to be back, have been unable to log in for quite a while..
Thanks
Susan
@Hidden Cottage

[levelbest]

Machine type? Big red or XT/XTS?

You didn't mention - if you run the same designs or other designs on standard material on the same machine with backing - does it trim every time them?