I've just had a chat with their really helpful support line. Restarting the EVSE (from the fusebox) and ensuring that the unit is switched using the key helps with the communication to their system, I'm told.
I'll try this first and see how I get on with it. The O2 signal in your area also affects it, as this is the GPRS signal it uses.
I'm sure I automatically signed up when I contacted Chargemaster for the install of my EVSE. If in doubt, call their friendly support line on 01582 400331.