I also learned that when changing thread color, be sure that the thread is snug in the the tension discs.
One day I ran out of bobbin thread and needed to take the spool that was on the machine...wound the bobbin, returned the cone to the machine, tied the end of the thread and pulled it through, threaded the needle and proceeded to quilt....half way down the row something sounded funny....loopy bottom...yes, i had not checked the tension disc..
Lesson learned.