Postgresql 9.6 HA issues
For questions like that, you'll have to consult the log files.

But the likely explanation is that the primary server was ahead of the standby when it crashed (this can also happen with synchronous replication). Then it cannot act as standby to the newly promoted server.

There are some solutions:


Easiest, but takes long: Reinitialize the server with pg_basebackup.
Faster, but more complicated: get the new primary in backup mode and use rsync.
Easiest, but requires that wal_keep_segments is high enough: pg_rewind the failed server.


You'll have to implement this in the failover scripts.

Why don't you use Patroni which does all that out of the box?