I assume these are floppy disks you are trying. If so, there are several possibilities that may be relevant to the long time storage of an Amiga:
1) The disks may be damaged. Try them on another Amiga to verify this.
2) The floppy drive may be damaged or the heads may be dirty. Get a known working disk from someone else (not a valuable disk, mind you) and see if that works in your drive. If it does not, you could try a head cleaner as a first resort (especially since it has been stored in a dusty environment).
3) There may be a drive head alignment problem. Does your Amiga write onto blank disks okay?
No doubt others will add to this, but I reckon you've got a reasonable start if you try my suggestions first.
