To set the film speed, you need to set the wheel on the film back.

That permits changing between backs loaded with different films "on the fly" without having to worry about adjusting the film speed on the camera.

As the film backs themselves are involved in the metering system, it is a good idea, if possible, to try another back to see if the problem originates with the back.

Does switching the metering mode from Average to Spot make a difference?

Are you sure the lens is set to open-aperture metering ("A") rather than to stop-down metering ("M")?

Although I think that if it was set to "M" it would give you the opposite problem.

Finally, are you metering manually, or using Auto-exposure? If Auto-Exposure, which mode?