DHCP - Invalid Request (Other - free products)

I am getting in my log a ton of invalid request messages. eventually after running the dhcp server for some time, people will complain that they aren't on the network.
what can cause this?
The web says Common reasons for this happening are the client trying to confirm a lease after it has moved to a different network, or after the least has in fact already expired
I dont think this is the case.

thanks,
Joel

sample from my log:
[08-03-05 07:40:56 AM] Known Host (DT102) Requested IP (192.168.1.61) MAC:00:0d:56:5f:5f:eb
[08-03-05 07:40:56 AM] Request invalid. (DHCP_Request)
[08-03-05 07:40:56 AM] Client:00:0d:56:5f:5f:eb tried to get IP:61
[08-03-05 07:40:56 AM] Request from 00:0d:56:5f:5f:eb denied. (DHCP_Request)
--------------------------------------------------------------------------
[08-03-05 07:41:01 AM] Known Client:DT102, MAC:00:0d:56:5f:5f:eb (DHCP_Announce)
[08-03-05 07:41:03 AM] Found Dynamic IP address (192.168.1.59) for Client:00:0d:56:5f:5f:eb
[08-03-05 07:41:03 AM] Assigned Client:DT102 IP address: 192.168.1.59 (DHCP_Announce)