I hadn't thought about Gary's idea about the stop-down lever resisting movement...but this could easily be the cause of your problem. If you unmount the lens, you should be able to move this lever very easily with your finger. If it resists movement, then you've probably found your culprit. You might try moving it a couple of dozen times manually to see if it loosens up. I don't know if these old linkages are lubricated, but it could be something as simple as the lubrication having gotten a little stiff over the years. If os, moving it a bunch of times manually might be enough to get the lubrication moving again.

Let us know what you find out.