I presume you are using Image Service Definitions and you are serving an ISDef file. In this case you can open the Reporter (Start,ArcGIS,ImageServer,Reporter). This will display as text all the request being made to the image server, the processing being performed and the errros. Alternatively access service provider logs stored in "\Documents and Settings\All Users\Application Data\ESRI\Image Server\Logs\ServiceProvider". This will detail any errors occurring. By looking at this it is likely that you will find that some of the rasters are not accessible.