Chan, I think pushing the pin on the lens doesn't close the diaphragm, it releases the diaphragm so it can be closed by an internal spring. This is why it doesn't feel tight. Even if it doesn't work the way I think, it would take very little drag to slow things down. When the lens is pointing up, the blades are probably lying on each other and dragging.

Check it with a different lens on that body to be certain it is the lens.