If it's the door split into four, then you have to go to the room to the right where there's tributes to four people. Click on one icon and there's a pattern on how the light shimmers. Replicate the pattern onto the corresponding door knocker
Um, maybe you aren't clicking it right? Holding down a click then releasing on a door knocker will create the big knock sound, the (-), while simply clicking will make a small knock, the (.). Each quarter should turn red once you've gotten that part right. Just a cute thing I noticed, I actually wrote down the pattern onto my hand for the door puzzle and I'm surprised we both wrote it in terms of (-)s and (.)s!