We have a 550 Color with network scanning. It scans just fine for 1-2 pages. But any more than that, it takes quite some time for the file to get delivered. For instance, 6 pages were scanned and it took an hour. The other day a few more pages were scanned, and it literally took 24 hours. I wouldn't say that scanning is failing, but it's taking an extensive amount of time to arrive at its destination.
I would make sure the date, time and timezone of the 550 and the bustled Fiery (if you have one) are correct. Also make sure the 550 ip is 100.100.100.101, subnet is 255.255.252.0, gateway is 100.100.100.102. This is for the 550 itself not to be confused with the Fiery settings.