Are you using Janome bobbin thread? My machine is very fussy! I usually turn the tension down a bit as well.
Try running some unwaxed dental floss through the upper threading just as you would thread the machine. Sometimes some bits of thread get caught where we can't see them or get to them. I can't promise this will help, but I hope it does.
LUCY HI. WITH MY JANOME I TURN MY TENSION DOWN TO 1. ANOTHER OPTION TO USE SAME THREAD IN BOBBIN. MY MACHINE HAS A HISSY FIT NOW AND THEN AND DOES THIS. CONCERNING THE USB WELL I DONT USE MINE. I USE MY CARD. SORRY CANT HELP YOU THERE,. HOPE THIS HELPS A BIT. HUGS PAULINE SYDNEY AUSTRALIA ;)