Device Won't Pick Up DHCP Reservation

I have just rebuilt my server, now running 10.11 and Server 5.0.4 and have come across a little quirk.

When I create a reservation for my Wifes shiny new 6S, I see these errors in the DHCP Service Log and the phone times out trying to get an IP

Oct 4 20:03:52 Server bootpd[12535]: DHCP DISCOVER [en0]: 1,6c:72:e7:1:7b:91 <Donnasrightarm>

Oct 4 20:03:52 Server bootpd[12535]: OFFER sent Donnasrightarm 172.16.0.22 pktsize 302

Oct 4 20:03:53 Server bootpd[12535]: DHCP REQUEST [en0]: 1,6c:72:e7:1:7b:91 <Donnasrightarm>

Oct 4 20:03:53 Server bootpd[12535]: ACK sent Donnasrightarm 172.16.0.22 pktsize 302

Oct 4 20:03:55 Server bootpd[12535]: DHCP DECLINE [en0]: 1,6c:72:e7:1:7b:91

Oct 4 20:04:05 Server bootpd[12535]: DHCP DISCOVER [en0]: 1,6c:72:e7:1:7b:91 <Donnasrightarm>

Oct 4 20:04:05 Server bootpd[12535]: OFFER sent Donnasrightarm 172.16.0.22 pktsize 302

Oct 4 20:04:08 Server bootpd[12535]: DHCP DECLINE [en0]: 1,6c:72:e7:1:7b:91

However when i remove the reservation I get

Oct 4 20:05:04 Server bootpd[12786]: DHCP DISCOVER [en0]: 1,6c:72:e7:1:7b:91 <Donnasrightarm>

Oct 4 20:05:04 Server bootpd[12786]: OFFER sent Donnasrightarm 172.16.0.50 pktsize 302

Oct 4 20:05:05 Server bootpd[12786]: DHCP REQUEST [en0]: 1,6c:72:e7:1:7b:91 <Donnasrightarm>

Oct 4 20:05:05 Server bootpd[12786]: ACK sent Donnasrightarm 172.16.0.50 pktsize 302

And the phone happily picks up 172.16.0.50 and connects no problem.....

I tried creating a static address from the this IP in the Server console (in case i screwed something up when creating the reservation) but that didn't work either.

I have 21 other devices with reserved IP's on the same server and so far none of them exhibit the same problem

Anyone have any ideas, or experiencing anything similar??

Thanks