Are you scanning a neg here or a print? I think the problem here may be that the original photo is "silvered" This is making the surface of the photo reflect and refract the scanner light all over the place, like a distorted mirror really, therefore no real image detail is being scanned. If you tip the photo at an angle in certain light and it looks silvery blackish on the surface then I think that is where the problem lies. It might help to scan the pic at various angles to see if you can reduce the effect. It might be possible to combine a few scans of the image which have been scanned at different angles to get the best from each area. However I think it is going to be difficult to do and the scans need to be identical in size and res so they can be combined as required.
This problem is generally caused by the original film being inadequately fixed and or washed in the wet darkroom. It may be possible to re-fix and wash the neg or print if you have that possibility, and dry carefully not allowing any droplets on the surface. Probably best undertaken by someone with experience in a wet darkroom.
Not very encouraging I am afraid but I hope it helps.
