Are your servers absolutely identical, or do they have different hard disk chipsets?
When you boot WinPE, if you cannot see the server hard disk on C: then it is very likely that your WinPE boot system lacks the correct driver for the hard disk chipset being used in that particular server.
While the server is booted, find the hard disk controller in Device Manager, and check the PNP ID. Compare this with the servers that can be imaged - my guess is that they will be different.
If I am correct, find the appropriate 32 bit Vista drivers for the chipset in the machines you cannot image, and add them to the WinPE boot disk. If you have any doubt about the suitability of the drivers, look for the presence of the PNP ID of your hard disk controller in the INF file that is part of the driver set.