Goodness I had forgotten about this issue. I used to have enormous problems in this area and it fell into 3 categories:

- the charger may say 125w but at peak can actually run higher
- there is a localised problem at your seat, next seat works fine
- the whole aircraft seems unable to take the charger, next identical aircraft has no problem.

So I swapped chargers to a lower spec one, measured maximum wattage and lo and behold all was fine. In my case I was having trouble with an OEM alleged 90w charger, however it actually ran at 140w in various circumstances and 747s couldn't cope with that. Some 777s were OK with it. I swapped down to a Hong Kong cheapo 75w charger and never looked back - it maxes at 78w. I certainly haven't had any problem on A380 (and I've been on all but one of them) or 787s. Obviously if you ensure your PC is fully charged before boarding, insert the power, leaving it 15 minutes before switching on, may perhaps help you. The problem often is that the charger surges a bit when the power is switched on to establish its equilibrium trickle level, thereby tripping the BA socket.