My question is did this happen on each machine after you had been sewing for awhile? If so you may have a small piece of lent somewhere that is messing things up.The fabric you stated was not top quality , some cheaper stuff does have alot of fibers in them.Also when fabric is made sometimes at factory we sprayed cleaners on spots that your neddle may be getting a slight coating and causing a ever so slight pull or gumming up your tension.