Auto Assigning dn numbers in cme 8.5 cisco 2821

I'm trying to test multiple phones and I have configured the router to auto assign numbers but not all phones pick up number despite being registered. Cisco 7911, 40s and 60s do but 7941s, 61s, 42s, 62s, 7970s 71s 75s don't. Am I doing anything wrong. Any help will be appreciated.
Edited. 7940s and 60s are not picking up numbers any more. I've used cisco 3725 as well. The problem remains.
Edited. 7940s and 60s are not picking up numbers any more. I've used cisco 3725 as well. The problem remains.
In my Lab
I have access to any cisco equipment I need
I have access to any cisco equipment I need
Comments
Reset 7900 Series IP Phones if Password is Set* [Cisco Unified IP Phone 7900 Series] - Cisco Systems
Thanks Pitviper, no errrors on most phones. Most of them are now getting dns except a few I had to erase their networking settings. I'll load the router with firmware tomorrow and reset the ones with errors to factory default. I did about 20 7945 without any issues today. All of them were auto assigned dns.
I have access to any cisco equipment I need