Knee stiffness 3 years after successful TKR - Any suggestions?

I had a TKR on my right knee three years ago. Except or a slight numbness on the lower outside of my knee I was extremely satisfied with the results. A few months ago I was side lined with a hip replacement followed by another TKR in my left knee. Since then my right knee continues to get stiffer and the area of stiffness is increasing. Although I don't have pain and my flexion is still over 130, it is stiff almost all the time. When I mention it to my doctor(s) they manipilate my knee, or give me an X-ray and tell me everything looks great. That's fine, but I' cant seem to be taken seriously get any answers as to what is going on? Is it scar tissue? Something else? Thoughts?