Make sure you have the wire cutters before the Brothel Mission, Chapter 3, and activate the opium trap. That's what I did and it popped immediately.
Hope this helps!
Here's my extended version on how to get to the box mentioned above (from the comments, as requested):
When you enter the main area of the brothel it's in the room to the right. You'll see a lovely woman trying to seduce a guard on the upper level. Once the guard walks away walk over to that back corner and there's a door you can open. Follow the linear path that's behind this door and you'll find the box you can use the wire cutters on.
It is kinda dark where the box is, so using focus might help.
Once you've cut the wires, exit back the way you came. There are 2 or 3 closets outside the door to this area. Enter one of them to save your game, then jump over the ledge trying to stay in the darkness. There's a lever to the right of the opium pump. If you've cut the wires then pulling this lever will knock everyone in the level out.