Did you open the chest? Are you in close up view? Did you then click on it?

There doesn't seem to be any trick to it. The walkthrough just says:

" Combination: After getting the clues and the numbers associated with the clues, enter from left to right: 8 5 2. Click on empty space on the dial to allow the arrow to move to the correct number.

Place the correct number inside the square. Hear a click. Click on chest while in close up.

Take the pistol with ramrod and the pistol spanner. "

Secret compartment: Look close at chest side.

See a hole at bottom center of chest front. Use the pistol spanner on the hole. The base popped out a bit. Click on it and Sylvie says something blocking it.

See a hole at bottom left corner of the left side of the chest.

In inventory, look close at pistol with ramrod. Click on the pistol at close up and see that the ramrod separates from pistol.

Use ramrod on the hole at left corner of the left side of chest.

Pull the front secret compartment and take the old manuscript.