I tried setting up the new Windows 2012 DHCP failover and got it working, but I couldn’t get some of my scopes that I’d imported from 2008 to 2012 working. If I built the same scope (or what I THOUGHT was the same scope) from scratch in 2012, I could configure failover.

I posted in the Microsoft forums but didn’t find resolution. However, one month later somebody posted the solution on my thread. My 2008 scopes happened to have both DHCP and BOOTP enabled, and you can’t enable failover on a scope set to both DHCP and BOOTP.