I have the same issue, my knee locks up, I straighten it to pop back in, it makes a pop/thud kind of sound, then I am back to normal. But there was a couple times in the past (including today) that it would lock up and wouldn't pop back into place until days later. I can walk on it, but I limp. Not much pain either, just uncomfortable. I saw your post about you popping it back in, what did you do exactly? I got it to pop back in unexpectedly when I was lying on my bed, waking up and turning over and felt it pop back into place.