Are you using the mono on a thread stand? The further you can get it away, the less tension it will exert on your piece.
I'm not clear if you've changed the tension on your bobbin as well as on top?
Granted, it is different than a domestic, but on my mid arm, I have to have quite a loose tension on my bobbin when I run MonoPoly (100 wt) on the top and King Tut (40 wt) on the bottom.
Watson