What is the status of the hard disks before you start imaging the first time?
I am totally speculating, but if the disks are totally wiped and do not come up as a drive letter at the point they are first imaged, then the boot information (such as boot.ini in XP) may not be correct for the end result. After the first image, the next time you boot the system, the hard disk now appears with a drive letter, and this time the image correctly deploys.
You mention that you are booting from CD, but do not mention whether you are booting WinPE or DOS. I suspect the former as DOS does not easily support USB devices, so once WinPE is booted, can you go to a command prompt, select C: and check whether you are seeing the hard disk of the machine, or seeing the CDROM?
If the machines have SATA disks, what are your bios settings for the hard disk when imaging? DOS does not understand SATA, so you would need to set ATA compatibility mode in the bios to make the hard disk resemble an IDE device rather than native SATA.