A few weeks ago I bought 2 canon fd lenses from a local classifieds. One was a Canon FD 50mm f1.4 S.S.C, the aperture ring didn't work. (the ring was able to move but the aperture stayed wide open).
I thought maybe the aperture was stuck because of oil on it. But after that I disassembled the lens, I found that it was not the aperture the problem but the canon fd breech lock mount. The ring inside was inverted so when you tried to turn the aperture ring, nothing happened because the "fork" on this ring was not aligned with the lock mount.
On ebay you can find a lot of canon fd lenses with "stuck aperture" but I suppose for a lot of them it's not the aperture itself but like my case, a breech lock mount problem.
Here is a few pictures to explained what I did:
(pour la version FR, me demander)
First you need to turn the silver ring's mount to have access to the screw: you need to push a small blade (see the picture) then turn to left the silver ring.
When you have removed the breech lock, observe the ring inside and if the "fork" on this ring can't go at the top (see my drawing) and if the black protuberance of the ring (on my drawing) is on the left of the warning sign instead of the right, it means that the ring is inverted and you need to put it back right.
You can use a screwdriver to push on the piece of plastic that is around the screw on the bottom (follow the arrow of my drawing with a warning sign) and in the same time move the "fork" ring to put it back in the right way.
When it's done you have to put back the breech mount. For that you have to align the mount with the fork. See the picture.
You can check if you have put it back in the right way by moving the two metal blades on the back of the mount, if they are not moving or if the iris is not moving, you have not aligned the mount properly.
Feel free to ask me your questions by commenting this post.