Barry Salter wrote:

It does seem to be a peculiar "feature" of the gate line at
Marylebone, in that a ticket issued at Marylebone will quite happily
work in any of the gates, but one issued elsewhere (even another
Chiltern station) will sometimes only work in either the old gates
(by the end of Platforms 1 and 2) or the wide gates.

Mine's a Chiltern-issue - and it's the old barrier line gates that I have
the problem with (although, to be fair, I can't comment with too much
authority on the new line, as I tend not to use them to exit the platforms,
as I'm generally heading for Melcombe Street, so try and avoid any crossing
manoeuvres with incoming passengers!). I've got into the habit in the last
couple of weeks of heading for the wide-aisle gate as a matter of course
now, although on Saturday night I didn't and again got caught by the old
gates (which is less of a problem when the station is quiet).