What keeps the action/forend relationship tight is a tight fit between the action knuckle and the forend iron radius. But that depends on a tight fit between the forend iron and the back of the barrel lug. The rear of the lug is often a slight angle, so that once the forend iron moves a tiny bit there is a loss of the tight fit.
Sounds like that fit may be really tight or the two surfaces are not polished. If that's the case try a little oil or gun grease on the back of the forend lug, then see if you can pull the tip of the forend out from the barrels while holding the button down.
This may help you home in on a diagnosis. Good luck.
