I haven't used monofilament thread in my bobbin, however, several years ago, I took a class with the owner of Heartbeat Quilts in Hyannis, MA, however, I can't remember her name right now (Sr. Moment or brain cramp - take your pick); but she said that she has used the monofilament thread in the bobbin without any problems at all. She also said that some machines might not like having it in the bobbin as well as the top. That's probably not much help, but I thought I would put my 2 cents worth in, but the quilt teacher is well known, I just cannot remember her name. I'm getting older and apparently so is my memory, so old it breaks down.