DHCPD not leasing IP’s to all clients

Okay, so i am running linux DHCPD on a Red Hat ES 5 Environment without any *real* issues…

The biggest problem that i currently have is that every so often, the dhcp server will not lease IP’s to clients…

Its not all clients that it wont lease to, it is Just some clients who wont get IP Addresses…

You can see the DHCPDISCOVER, & DHCPOFFER requests showing in the debug, but the clients Just are not accepting the ip’s.

(Remember that this is not all clients, say out of 200 clients, only 12 or 13 clients get this problem).

Any idea’s as to what/why this may be?
it is starting to turn out to be a big headache.