You probably have to set the aperture manually on the lens, because it might not have CPU contacts. I think changing the aperture on the camera will probably have no effect, so just try twisting the aperture ring to wide open.

You usually have to set the aperture at the minimum setting (highest number) so that if the camera is choosing the aperture, it can stop down to the required value.

No good having it set at f2.8 if the shot needs f11.

If the lens is very old, it might have a manual iris that you have to set yourself on the lens. The lenses that stop down automatically will have a tiny peg on the mount that a suitable camera can operate. Gentle pushing of this peg while looking through the lens will tell if it is stopping down - set it wide open first.

If you have a depth of field preview button, this will also have the same effect with the lens on the camera if the body matches up with the lens.