It could be that the surgery missed something and there is a phycial block to movement or it could be that the patient is subconsciously blocking the movement. That said surgery is often the problem in later stages where normal range is not achieved. The first thing to do is find out this answer.

Closed chain exercises are useful but if the deformity is flexion, then active concentric knee extension exercises might assist with the relaxation into eccentric flexion. You can do the same for eccentric hamstrings work (prone with assistance into flexion) where the eccentric work may let them go into more flexion. Numbing up the area with ice prior and during might assist even more if this is a brain lead restriction rather than a physical one.