From what I saw the reason is that the battery drains even if external power is on and the ammeter does not show a negative load on the battery! Watch the battery voltage going down while you prepare for engine start. Below a certain voltage there is no way to start the engines.

If you have a registered copy of FSUIPC give the battery unlimited life, that get's this problem solved - Mark I guess that's why you don't see the problem as well.