I've noticed some of those TIMs in the bus provide just enough current to trigger the "charging" symbol but don't actually outpace the [presumably] heat effect at high cabin altitudes.

Printing all the necessary pages and taping them to the glass on the iPad has yielded the longest battery life.

I've found that turning the TIM off, then back on, in those cases gave me a better charging current.

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Or literally pulling the cord out of the iPad and then puttin it back in sometimes starts the charging as well.

OK...what's a TIM? (Besides the Enchanter in "Holy Grail").

Transformer Interface Module?

I think it has something to do with the power swap on engine start. I've found it charges until engine start, then you have to turn it off and on again to get it to charge things.

One of the plugs provides less current than the other. Make sure you are using the correct one.