i defeated scabban and pushed the button on the wall. the underwater gate wont open. is this a bug or am i missing something?

some time since i was back there. I seem to remember two gates one in the asylum which was more like a grate underwater which required a button to be pushed somewer and the other the gate back to the pool were you meet Raskillion? (in fact two gates here) were you pick up the key to open the tombs. There is a lever on the wall (underwater) to open these.
Which one are you asking about. If its the one in the asylum you may have missed a button lever somewere.

There's a button in the room where you meet Scabban that opens the grate to the sewers.
There's a lever on the wall of the underwater passage that opens the grate to get out of the sewers.
There's a button on the wall in a room just down the hall from the room with the grate entrance to the sewers that I never found a use for. Nothing seemed to change anywhere no matter how many times I pushed it.

most likely a bug pushing that button the grate should open, I even had it work before talking to scabban...