Lew 1 wrote:

Luton Airport Parkway.

And as of last week, I still can't get through the gates - yet other gates
let them through fine. (Including Blackfriars LUL).

That's even more wierd - my tickets from last week were also issued
there, although they were purchased from NXEC via their website earlier
and I merely collected them from a TVM. Admittedly over the past 18
months most of my tickets have been issued in London (even though I
travel to/from LAP often, whose TVMs have only been ToD-enabled
recently) given, IMX, the covenience of buying and collecting tickets
once a week rather than having to buy one every single weekday. (And
yes, my travels are such that they are cheaper than season tickets.)

If you haven't already done so I'd suggest contacting FCC Customer
Relations, and in the meantime I'd also suggest using City Thameslink,
which is (approximately) a mere 200 yards north of London Blackfriars at
the Ludgate Hill end. Depending on your circumstances it *might* be
worth buying your tickets at another Thameslink station (e.g. Luton),
unless you've already tried that to no avail.

Finally, if FCC don't resolve the problem within a reasonable amount of
time (after you have contacted them) then contact LondonTravelWatch or
Passenger Focus.