Trouble with Unsharp Mask

I am using the old version of Photoshop that can be downloaded for free from the Adobe website. It is Adobe Photoshop CS2 Version 9.0. Until recently the Unsharp Mask tool was working well, but now it has stopped working and I don't know why. I have made sure that I am clicked onto the background layer. When the Unsharp Mask tool box pops up, the preview box is checked. As I adjust the Amount, Threshold or Radius, it changes in the preview box, but not on the full image (it used to adjust both). I've tried adjusting based off of what I see in the preview box and hitting OK, but it does not apply it to the image.  Has anyone come across this problem and figured out how to fix it?