Wal fetching standby refuses to promote

This was an atomic snapshot put in recovery and happily reading WALs
from the repo.

It's consistent and allows read-only querying. It's replayed a few days
worth of WALs and was previously running with a recovery_target_xid =
$some-xid which apparently was never hit and/or aborted.

It would not promote then, after pg_ctl promote. The 'promote' file did
get created in pgdata.

Now it's running again with recovery.conf that contains just...

standby_mode = on
restore_command = 'cp /somedir/%f %p'

Still it fails to respond to the issuance of a promote.

There is no trigger file config's in recovery.conf and we have never
needed to do same since pg_ctl promote usually works.

Any idea what's causing this?

Thanks

version
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PostgreSQL 9.6.6 on x86_64-pc-linux-gnu, compiled by gcc (Ubuntu 5.4.0-6ubuntu1~16.04.4) 5.4.0 20160609, 64-bit
(1 row)

--
Jerry Sievers
Postgres DBA/Development Consulting
e: postgres(dot)consulting(at)comcast(dot)net
p: 312.241.7800