Had the same problem with two cards (myself & partner) which should
have topped-up when we touched in at "King's Cross (Met, Circle, H&C)"
on our day trip to London on 10 Sep. Didn't realise there was a
problem until we tried to exit King's Cross heading for our train home
and one of the cards wouldn't let us through as it was out of credit.
Logged a complaint last night via the TFL web site, but no response as
yet. We're visiting again before the automatic 7-day refund kicks in,
so may have to resort to ringing the help line to sort it out.