I have similar experience. Only two years ago my father had asked me to scan a good number of his pictures taken on Kodachrome and Agfa when he was a yound unmarried man working in the Europe, the Middle East and Africa in the early 60's.
The Kodachrome are all still brilliant colour, the Agfa not so. While they all seem to project sharp many of them have not been able to be scanned. I put it down to what the images are projected on. A painted wallpapered wall, a sheet or even on the rare occasion a projection screen, none of which has had a brilliant surface for determining if the image was sharply focused or not. Dust and finger prints are also a big problem as Dad did not think to not let my brothers and I as kids handle all of the slides in and out of the carriers.