IME, this feature is SUPER sensitive.
I tried to charge my daughter's iPhone recently, on a trip, and failed miserably. Both charge wirelessly just fine, and I could get it to start, but after a minute or two it'd turn itself off. Gave up after maybe 6-8 tries.
Yes, both phones were in cases (and both wireless-charge fine in them), so I guess it's possible that the combination of two cases was simply too much, I suppose...

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