I bought a used Boss and the Primary tension wheel cups were reversed (facing each other). This made it impossible to get the right needle tension.
Check to make sure that when you stitch, the Primary Tension Wheel turns. Rule of thumb for thread tension is: The bobbin thread tension is half the tension of the needle thread tension (Bob at Tippman told me this).
Keep trying and one day it will all click just like on the island. Remember nothing good comes easy.
Semper Fi, 1969-1972