I had a weird issue yesterday. I had a Cisco 1800 series router that was configured to hand out DHCP and up until a few days ago everything worked fine with it. Then suddenly out of the blue it still handed out DHCP but did not hand out the DNS-Server option even though it was configured.
I tried removing and re-adding that option and even rebooting the router but that didn’t resolve the issue. In the end I blew away the entire DHCP pool and re-created it and that resolved the issue.