Why can't some of my machines boot to my Lion install USB drive?
It sounds like the USB drive doesn't have the drivers to boot the other macs. :-(

The fastest path forward is to simply erase install a basic Snow Leopard OS onto the failed macs. While this is happening, do download the 10.6.8 Combo update to your USB drive just in case it's needed. 

Don't bother running the updates unless the Lion installer forces you to get to a higher version than your 10.6 installer delivered.

Once you boot into Snow Leopard - you can try again to mount the USB and execute the Lion upgrade package. I don't know if it runs well from the USB or needs to be copied to the internal boot drive.

There's a little uncertainty in my brain - so I don't want to write too much without making sure this makes sense to you.



As an alternative - you could try instead to transfer the recovery partition, but this may not be universal (include the drivers the older macs) either.

There is a step-by-step recipe here for copying any bootable volume to one file on a USB drive.

If you are curious or feel it's worth a shot, image the recovery partition from your Lion mac.

You should be able to boot from DVD and use disk utility to make an equivalent partition on the "non bootable" mac and drop the recovery data to get a minimal bootable system and avoid a full Snow Leopard install.

Unless you are familiar with Disk Utility and the steps to capture, the reinstall option might be more likely to succeed on first attempt. I certainly don't know if this partition is customized by Lion and not universal so I've made it an aside for the curious.