After spending an hour trying to find out why my slave didn’t attempt to sync with my supermaster server (my domains were set to MASTER, axfr worked from the slave, etc..), I came across the fact that even though your server is authorative and will handle requests, it will not pretend to be an actual MASTER server unless master=yes is set in the configuration file.
One reload and suddenly all the domains synced magically.
Do check that one if you’re not seeing any notify-requests or any entries in your logfiles at all when updating or requesting a sync (through pdns_control).