You may have what's called a contracture. Muscle or joint tightness keeps the joint from moving to the end of its range. If you can't straighten it out, it's called a knee flexion contracture because your knee is stuck in flexion.
There are other causes for contractures. Sometimes there's swelling in the joint. The muscles on either side of the joint may be imbalanced and fighting against each other. In other cases the implant is not aligned properly. Scar tissue can also pose a problem.
By six months you should have all of your motion back. It's best to discuss this with your orthopedic surgeon. A short course of physical therapy may be needed. If that doesn't work then sometimes a second operation is needed. Don't wait because the problem isn't likely to go away on its own. Early treatment usually results in better and faster results.