I installed knockd on a Pi2 and there seems to be an issue with the knockd service. On boot it starts and I can see it loaded but it doesn't react to any knock.
The service file is in /usr/lib/systemd/system/knockd.service
I added ExecStartPre=/bin/sleep 5 right before the ExecStart line and that seems to fix it. It waits around 5 seconds before starting. Might be a timing issue, but it works like this.
I'm making this thread mostly for anyone else having this issue.