With the forend off you will see a small rod on the right side protruding out of knuckle that should stick out quite a way. If it does not not move freely when you press it firmly then it needs to be removed and cleaned and lubricated. That small rod is what locks the cocking hook in place with the forend on and also releases the hook with the forend off so the barrels can be removed. A very small screw on the flat of the action is what holds the rod in place and needs to be removed to withdraw the rod for cleaning. The tiny screw head is intimidating and you’d better have a perfectly shaped screwdriver to remove it. An alternative you might try first is to squirt some Kroil or similar fluid on the rod and work the rod back and forth to see if it loosens up and works properly.
Another suggestion. Along with being sure the rod is free and working easily (it does have a coil spring in it), be sure the cocking rod moves freely. There is a small hole in it where a lubricant is supposed to go. I’d again suggest something like Kroil to free it up. The gun really needs a complete stripping and cleaning. Do NOT take a file to any part—it is already perfect.
PM me if you have questions.