It's nearly impossible to say without seeing the actual photos.
Here are some general things to think about though:
Unsharp increase local pixel contrast to make things appear sharper as their highlights are "stronger" once it has been run.
Any filtering will generally cause some loss of the original detail of the document. Even if things become clearer, we are still changing the original with syntetic methods. What this boils down to is that the same procedure doesn't always work right even if we think it should 
Sometimes using a high pass is a better method of "sharpening" than unsharp. Duplicate the layer. Run filter > other > high pass at a pixel or two (just until you start to see lines appearing in the grey, just takes a little practice to predict). Then set your blending mode to something soft like Overlay.
Learning to use the "custom" filter is a major headache, but worth the effort. I'm not a master at this, but I've worked with it a bit. This is one of those black magic type filters that just takes some doing. Anyone else have some good information on "custom"? I don't have a thourough understanding of how the engine works, other than I know that it changes the pixel contrast and (i believe luminocity) based on the matrix input.
Sometimes the problem with your photos isn't "sharpness" but overall cast removal, or color correction. If things aren't looking as sharp as you want, make sure your neutrals are balanced and your histogram looks like what you'd expect it to. Getting a skintone and a greyscale ramp are good guides to help with this.
Hope that helps.
