After trying several types of thread tensionsers from manufactored sewing machine tensioners to home made springs based on mouse traps or rod tips, I finally have decided that the best is also the simplest.

I run the thread thru a phone book and place a bowl full of pennies on top. The tension is easily adjusted by adding or removing a handful of pennies. It has an amazing range of adjustment and won't fray fragile silk threads.

That said, you might also check your spool of thread. Once I found one that had small razor along the spool. It looked like someone had accidentally nicked the spool while opening a carton or removing a label.

The thread was fine for several revolutions, then it would spool into the damaged area and break with almost no tension. I solved the problem by unwinding several yards of material to get past the damaged fibers.

Another thought, it makes a difference on my screw down type tensioner if the spool is mounted with the thread winding clockwise vs counter clockwise. Mounted the wrong way it has a tendency grab the wing nut that holds the spool in place and tighten it.