Phil,
As Ian has said, some of the modern 'smart-charger' packs can't 'see' the Raspberry Pi properly so don't supply sufficient current at boot up, even though they may be rated at a maximum of more than 2 amps. Can you post a link to the power pack you are using, or give us the name and model so we can check the spec.
Regards
Peter
If you are running a standard PilotAware, rather than an OGN Relay station, Lee will perhaps move this thread over to the Tech Support Forum for us, to avoid confusion.