Another reason why difficult safety engagement occurs, is that once safety/bolt components have been installed, the mainspring coils are sometimes bound against one another on the cocking rod as the safety is being turned over. (compressed together)
The amount of clearance on the safety cams and cocking piece cuts/slots before they are picked up, cannot exceed the amount of clearance throughout the mainsprings coils very much or it will turn hard.
Grinding the ends of the springs ends flat will sometimes ease the tension as long as the turns/ diameter of the spring are not too far out of whack.