Just an off the wall guess......are the strikers (the hammers) down, as in both sides uncocked, before you try to assemble the rifle.
If so, you are working the forend against the cocking levers when trying to attach the forened.

Before you try to attach the barrels use the forend to “cock” action (hold it against the frame knuckle just as if it were being broken open after a shot) only the barrels arent attached yet.
This eliminates the mainspring tension.

Just maybe is what you are working against?
My 2¢
- Mike