I have an annoying problem with SBS 2008. Everything was fine till I authorized external DHCP running on Linksys router. Now I have that kind of situation where I think everything must be fine but it isn't. Now DNS records of computers in domain aren't updated. The corresponding option is enabled in DHCP server's configuration, however it has no effect. I don't see anything in logs, too. I've tried everything - even removing that DHCP server from network. Now some computers get IP from DHCP, and some don't. It's very strange thing. Also, I've checked if it was a hardware problem and it must be something different. I think it's releated to that empty logs. Ah, and before authorizing that DHCP on Linksys, that on SBS 2008 was crashing with message about other unauthorized DHCP servers in the network, but strange thing - everything was fine. Any ideas?
Please see my question, I think I had the same initial issues as you where on 2008 DNS Changes to DHCP assigned hosts were not seen/updated. The commented KB fixed this issue.
In terms of DHCP not assigning new leases. Check to make sure your scope is infact Authorised, the DHCP server service is started. Windows WILL let another DHCP server win everytime and stop it's own service.
It is worth checking your event logs to see if the DHCP server is reporting any warning or errors, you might also find that your scope is full.
I hope this helps, let me know if you have any further issues.