There are many possible causes of continued knee pain after joint replacement. The plan to do revision surgery suggests the surgeon thinks the problem is coming from the knee or the implant itself.
Sometimes the implant isn't in the best position and the knee biomechanics are off. Pain, swelling, and stiffness can occur. There may be instability at the knee. Tension in the soft tissues such as muscles and ligaments must be carefully balanced during the operation. Failure to do so can lead to pain from pinching of the soft tissues or giving way of the knee.
In other cases too much scar tissue forms and the joint starts to lose motion. The result is usually pain and stiffness. The implant can also loosen, get infected, or even fracture causing similar problems.
Ask the surgeon for an explanation of what's wrong and what is the treatment plan. If your mother has had a painful knee for years she may need some extra rehab to restore muscle strength and motion. An exercise program is often a good idea before and after revision surgery.