clients keep losing dhcp address

We are a Windows 2008 domain environment.
Our dhcp range is 192.168.1.50 to 192.168.1.200 subnet mask of 255.255.255.0

We have basically run out of ip addresses
We changed our subnet mask to 255.255.252.0 which now gave us almost 750 more ip addresses.
We then changed our dhcpk scope to 192.168.0.20 to 192.168.0.240.

After rebooting Windows 7 client the computer did not receive an address.
After an ipconfig /release and renew it received a new address.

After rebooting the clients lose the address again.

Tried multiple clients with same result.

Removed scope and recreated the original scope still using the new subnet mask abd everything works fine.

Is there something needed if dhcp server is 192.168.1.20 but giving out addresses on 192.168.0.x  
All the ip addresses fall within the subnet mask.
Any thoughts are appreciated
EkuskowskiAsked:
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rharland2009Commented:
Just to clarify, you said after deletion and recreation of the scope you have normal operation, right?
I think the problem may have been with some orphaned info inside the original scope.
Historically, if I've changed the mask on a DHCP box I have to delete and recreate the scope to get normal operation.
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vivigattCommented:
Use this site:
http://www.subnet-calculator.com/
I assume that you created a scope of  192.168.0.20 to 192.168.1.240 or even 192.168.0.20 to 192.168.3.240 (and not 192.168.0.20 to 192.168.0.240 as you wrote).
You may envisage that your new scope overlaps with the old one so that existing clients can still use their previous IP addresses. Windows clients will try to reuse such an address (preferred IP address).

These articles might help:
http://superuser.com/questions/470585/how-to-force-windows-7-to-ask-for-a-fresh-ip-address-from-dhcp-server.
http://mikepeth.blogspot.fr/2012/02/windows-dhcp-running-out-of-ips-and.html
 
You could shorten the DHCP lease to a small amount of time, tweak the Cleanup Interval and Grace Period and see if this is any better.

If nothing works, record a trace of a DHCP transaction that is not working (use Wireshark or MS NetMon), filter on UDP 67 and 68 and post the trace, we can then examine it and see what is going wrong...
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EkuskowskiAuthor Commented:
I'm accepting these two answers because it gets me in the right direction,  I'm not sure when we are going to try again with the scope change.
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