Why won't the ISO settings change on Nikon D5100?

So I borrowed the 5100 for the weekend, and I'm having problems with the ISO settings. I have the camera on manual mode, the ISO is also on manual, but when I change the ISO level from 100 to 6400 there's absolutely no change. I don't understand why this is when both the camera and ISO setting are in full manual. Can you help?

When you set the ISO on a camera, what exactly are you expecting to see?

You should see the lightmeter showing you exposures that will be shorter as you increase the ISO.

It seems clear that you do not have a full understanding of how you are to balance ISO, shutter speed and lens aperture.

Here is a link that should help

Verify that the ISO hasn't changed in the status screen in the viewfinder or at the rear LCD. If you think you're doing it right, the camera may be broken. If you are unsure, download a copy of the manual from Nikon's website and check how to correctly set ISO manually. Take note that with some entry-level dSLR models, setting ISO manually gives you two choices, a range where ISO is still set automatically or the option for you to select ISO yourself. This can be seen when there are two ISO values on the rear LCD - one is in a range while the other is just one value. You must go down to that singular value and change the ISO there.

Use the light meter in your camera if you want to use Manual mode, and learn how to balance the exposure.

The only thing that changes in the viewfinder when you change the ISO is the needle of the light meter to indicate whether the proper exposure has been achieved.

Maybe the Auto ISO Sensitivity Control is set to On. When set to on, your camera will override your ISO setting if the camera can't correctly expose your image using the ISO that you set.
Go into the Shooting Menu and, switch the Auto ISO Sensitivity Control off if it is set to on then see if it makes any difference.

Check that Auto ISO is OFF.

Even if you are in Manual exposure mode, Auto ISO still works if it's on and can throw out your expected settings.