Rengar Stealth Indicator not working properly

I was playing a game against a Rengar and the exclamation mark would not pop up above my character whenever Rengar was stealthed. Many times he just popped up out of no where and I had no idea he was nearby. For reference, no I was not near a bush when this would happen. I would be in the middle of the lane and then Rengar would be next time out of no where. Has anyone else had this problem? It's really annoying for this to happen when I have to play against him.