DHCP Reservation made in DHCP server showing up as “inactive”
In order to get the reservations in the DHCP server to be shown as Active, the machines you’ve assigned reservations for need to use DHCP for ip address assignment (not static ip).
It looks like you already tried that, but if you change to DHCP on those clients, run ‘ipconfig /release’ and ‘ipconfig /renew’ on the machine, after a refresh in the DHCP console, they should show up as Active if all is configured OK.
Read more about DHCP reservations here:
http://technet.microsoft.com/sv-se/library/cc779507(v=ws.10).aspx
This information taken from a forum, here:
https://social.technet.microsoft.com/Forums/office/en-US/e901f44f-65a9-4680-9aa9-d2b3c3e2cd22/dhcp-reservation-inactive-help?forum=winservergen