Remove the bottom cover (5 screws), and lift off the sliding "clam shell" cover -- careful! There is a loose stainless steel roller that sits free under this cover! Don't lose it, or you'll have to make a replacement!
Now remove the front cover. There is one black screw inside the back of the camera, and two silver ones in the front.
Watch as you move the aperture slide up and down. See how it is held by two screws in two slots of the slide? If the slide was ever forced, it may have bent one of the slots. If so, try gently prying the slot so that it's even again.
What may have caused this is if the top cover was removed, then replaced with the aperture slide NOT set to the full UP position. There is a detent spring at the top of the aperture slide's travel to make it difficult to push it to activate the flash. This detent spring will often catch on the glass window of the rangefinder as the top is installed. If it does, and the aperture slide is forced, it will bend so that it's movement is difficult, as you've described.