The bottom line is that if the unit is getting power above 10V (as shown on the M line of the voltmeter that you can pull up via the INFO>VMETER info item, and it's on, then the internal battery should charge.
It sounds like your unit was perhaps running of its own internal battery, but, if that had happened after it was charging successfully, it would have turned itself off after 60 seconds as it didn't receive power from master. Now if you turned on the unit via button 1, and then applied the master power, but there wasn't actually power there, then that would cause the discharge behavior you're seeing. But you thought you saw the voltage rise. So not all is adding up here.
Our suggestion. Reinstall the battery, get power on the unit for an extended period of time (aircraft battery hooked up to a battery charger is a good way to do this), and see if that produces results. If not, get in touch with our tech support directly.