Your active participation in rehabilitation is an important factor that sometimes minimizes stiffness in a new knee joint. But there may be other reasons why your knee is still stiff.
Some people have a tendency to form a lot of scar tissue after an injury or surgery. When this happens after total knee replacement, the added scar tissue can produce stiffness in the knee. Likewise, people who go into surgery without much bend in the knee commonly have problems getting full knee bend after surgery.
Bear in mind that not all problems of stiffness are due to patient factors. Errors in the way the surgery is done can affect whether the knee is stiff after surgery. Improper alignment in the parts of the new prosthesis can lead to tightness in the hinge-action of the joint. Other technical errors during surgery can also contribute to tightness. Complications can result after surgery if there's infection or if the prosthesis has become loose or broken.
Be sure to discuss your concerns with your doctor.