It's not your fault that your ROM hasn't increased as fast as your surgeon wants.
All those stories about scar tissue and the need to work hard to keep it at bay are total myths. I don't know where they started, but I sure wish they'd go away, because they're the cause of so much heartache.
I think that perhaps you initially worked too hard at the exercises - not your fault, as you were just doing what you were told - and doing that irritated you soft tissues, that were just trying to heal. You did the right thing in cutting back the exercises.
I'm fortunate, because my surgeon doesn't believe the myth about "scar tissue". He doesn't allow any PT at all for the first month after a knee replacement. He says your knee needs that time, to start on its journey of healing. For that month, we rest, ice and elevate our leg, and walk around the house.
After that month, we just go to PT once every 2 weeks, where we are shown a few new exercises to do at home.
His patients all do well and achieve good ROM, as I did, and he hasn't had to do a manipulation to help with ROM for the past 4 years. I think that speaks for itself.