Give us an example of a fairly bad one.
Do they have a red cast?
Are they reversal film or raster prints? (They did that in the 60's!)
Do you have a film scanner that can hold framed slides?
Are they "wet" emulsion-to-emulsion copies? (Emulsion on wrong side: wrong autofocus in scanner, put in backwards and flip in PS).
Are they realy 800?