I've been having an issue on my Dell 1600N All-In-One printer where the network scan would initially work and then it would stop working. Opening the network scan manager and trying to find the scanner fails every time. After a searching the Dell forums it turns out that the problem is that I had an additional network adaptor configured as an MS Loopback Adaptor. As soon as I disabled this, the scanner is found. In my mind this is only a temporary workaround, but at least I know the issue and am able to resume scanning.