The CT cotton thread is 50wt 3 ply. You need a size 90/14 needle or bigger for the thickness. If you aren't changing the tension when you change thread then that says a lot. Other brand of thread is probably a different weight, different cotton, so it sews with the size needle you have and the tension setting you have. Every once in awhile I'll get a spool of thread that just won't sew even if I had no problems before with it. CT will replace their spools so call them.