Trouble getting network printers to print

[Ronald Cogen]

I've set up the following network printers served by a HP ML370 running SuSE Enterprise 8:
HP 4 plus (Netport pro)[192.168.5.103]
Xerox N24 Hochleistungsdrucker [192.168.5.100]
Xerox N17 (Netport pro)[192.168.5.101]
Xerox LP4: Farblaser [192.168.5.102]
The printers are connected to an HP 2626 Switch. Two of the printers go through an Intel Netport Express Pro. The ports on the switch are set to Auto Detect/Flow Control Disable. To begin with, only 3 of the 4 printers appear in CUPS (?). Otherwise I can ping and telnet on to them.
Before setting up using CUPS I tryed to set up a direct TCP/IP print connection. For some reason I can (sometimes but not always) get data to one of the available printers (Xerox N17)through Port 515 but not through 9100 or 631. I can also log on to the printers via the web interface http://ip_address.
I'm completely baffled. Why don't they just print? Where can I trouble shoot?