I've noticed that when i straighten my leg out & come to bend it - the cap locks in place. If i push it (right kneecap) from the outside edge, towards the inside, it sort of 'pops' (that's the wrong word as you'll be thinking of an audible "pop", but there's a bit of resistance, and then it just gives). When it is pushed to the left, i can then bend my knee no problem.

It still does this locking every time, but i've figured out how to stop it every time. It's just not very practical having to manually push your kneecap across.

Why would this be?
Is it common after this type of surgery?
Chances of this problem disappearing?
Could it be linked to the loss of strength/muscle wastage in my thigh?