After aggravating an old back injury in work 8 months ago, I was seeing a Worker's Compensation doc who basically told me the same thing, but he worded it a little bit differently. I was in PT for 4 months, and after being the good little patient that I was trying to be, they put me through some kind of stress test to see how far I had progressed from the day I limped into the clinic.
In his words, I had regained as much "flexion" as I was going to, and since my condition is degenerative, that basically there was nothing more that they could do for me with PT. Then he released me back to work with so many restrictions that it was humanly impossible to function in my old job in that capacity.
In a nutshell, he told me to continue doing the stretches & exercises that they taught me in PT but not to do anything unnecessary to aggravate my condition further. So I guess, in his own way, he was telling me that I had reached my own "plateau."
4 months since being discharged and I'm still in pain, out of work & trying to put the pieces of my life back together. Good luck, I hope that your recovery prognosis is better than mine has been, I wouldn't wish this nightmare on anyone.