I tried to reply to this yesterday but ended up getting duplicate messages and was unable to edit them sufficiently. So here goes again.
Please make sure any bolster of not in the bend of the knee. There are good reasons for this but not necessary for this part.
There can be many reasons for the patella to snag and make it difficult to bend your knee I'll briefly mention a couple but rest assured that is fairly common.
The quad could be tight restricting inferior movement of the patella into the trochlear groove of the femur. The lateral peripatellar soft tissues(retinaculum, ITB,etc) could be tight, swollen, and have restrictions limiting the normal inferior and medial movement of the patella into the groove.
You probably need lateral thigh soft tissue flexibility in a manner that doesn't produce pain or increase swelling and some medial thigh muscle recruitment of the hip Adductors. If the femur lays rotated out this can exacerbate tightness causing a pull on the outside of the patella. Normally we also have some medial thigh muscle strength to counteract that to some degree but after surgery that declined significantly. You can have an imbalance between tightness on 1 side of the thigh and weakness on the other.
All too often people focus mainly on the knee extensor mechanism and not near enough in the hamstrings and surrounding thigh muscles.
I have seen the biceps femoris hang up on the fibular head but that is not common and from your description probably not even close to the problem.
You may have to manually move your patella before movement until these issues are resolved but that shouldn't be too long.