PowerDNS master server doesn't notify

PowerDNS master server doesn't notify

I have a PowerDNS based master nameserver with 3 slaves. Zone transfer
works through AXFR (automatically once every hour it gets checked by the
slaves). When I change a record through our panel the notified_serial gets
updated correctly as well as the serial in the SOA record.
The nameserver uses a MySQL backend. The slaves don't get notified when
changes occur. Nothing is logged when it was supposed to send a Notify.
When I force a notify (like: pdns_control notify example.com) the slaves
get notified properly.
The config is as follows:
master=yes
setuid=pdns
setgid=pdns
local-address=xx.xxx.xx.xxx
allow-axfr-ips=xx.xxx.xx.xxx
use-logfile=yes
log-dns-details=yes
log-failed-updates=yes
logging-facility=0
loglevel=4
launch=gmysql
gmysql-host=localhost
gmysql-user=xxxxxxxxxxxxxxxx
gmysql-password=xxxxxxxxxxxxxxx
gmysql-dbname=powerdns