To really troubleshoot this you would need the Device IDs of the devices in question. In 9.5 SP1 (which I know is apparently not an option right now) there is a parameter in HiiClient that will dump all the hardware information on the device. The parameter is /previewall.
In 9.5 flat and earlier you would need to go into device manager on one of these HP 840 devices and find the device. Double click it and go to Advanced. From there find Hardware IDs. Post the Hardware IDs here and I can help find the appropriate WinPE driver.
Thanks!