PowerDNS slaves not updating after being notified

We were experiencing this and it turns out that the target of the DNS notification message was actually refusing the message.

Notice the "notify Refused" below. Substituted fake server and zone names.

    # tcpdump -v -r notify.pcap
reading from file notify.pcap, link-type LINUX_SLL (Linux cooked)
00:00:33.210137 IP (tos 0x0, ttl 64, id 0, offset 0, flags [DF], proto UDP (17), length 61) master.dns.server.46861 > slave.dns.server.domain: 49437 notify SOA? zoneinquestion.com. (33)
00:00:33.236488 IP (tos 0x0, ttl 55, id 17352, offset 0, flags [none], proto UDP (17), length 61) slave.dns.server.domain > master.dns.server.46861: 49437 notify Refused- 0/0/0 (33)
00:00:36.244057 IP (tos 0x0, ttl 64, id 0, offset 0, flags [DF], proto UDP (17), length 61) master.dns.server.46861 > slave.dns.server.domain: 48449 notify SOA? zoneinquestion.com. (33)
00:00:36.269682 IP (tos 0x0, ttl 55, id 17353, offset 0, flags [none], proto UDP (17), length 61) slave.dns.server.domain > master.dns.server.46861: 48449 notify Refused- 0/0/0 (33)
00:00:36.519361 IP (tos 0x0, ttl 64, id 0, offset 0, flags [DF], proto UDP (17), length 61) master.dns.server.46861 > slave.dns.server.domain: 65128 notify SOA? zoneinquestion.com. (33)
00:00:36.544391 IP (tos 0x0, ttl 55, id 17354, offset 0, flags [none], proto UDP (17), length 61) slave.dns.server.domain > master.dns.server.46861: 65128 notify Refused- 0/0/0 (33)

Captured this output on the master with the following:

tcpdump -U -i any -w notify.pcap -s 1600 host slave.dns.server