There are many, many possible reasons for a failed joint replacement. Studies show that poor motion before the operation has a strong link with poor motion afterwards. Younger women actually do have poorer results compared with other ages and gender.
Even the patient's personality has been identified as a possible contributing factor. Patient's who do not tolerate pain well and especially those who do not follow the rehab program have increased rates of knee stiffness after a total knee replacement (TKR).
There may even be a genetic basis for your poor results. Tissue sampling and analysis has shown chemical changes during the healing phase in patients who end up with stiff joints. The body doesn't clear the joint of inflammatory cells. There's a build-up of too much protein called reactive oxygen and nitrogen species (RONS).
Scientists are studying ways to prevent this build-up as well as stiffness from other causes as well. There's still hope for you now with continued rehab. Your age is in your favor there. It may take longer and may require persistence on your part, but restoration of normal to near normal motion is possible.
If you have already done all this, then an operation to manipulate the joint and break the scar or fibrosis holding the joint may be needed. If that doesn't work, then a second or revision operation can be done. Your surgeon will be able to advise you what treatment might work best for your situation.