DNS Question

We currently implmenented a new firewall to track internet usage on our LAN. The problem we are having is that it is logging the wrong names of the PC’s in respect to the IP addresses.

Example:

PC John_Doe is a computer that is being logged as Jane_Doe. The thing Jane_Doe hasn’t existed in a month. Also, if you ping Jane_Doe and John Doe, they both reply back with the same IP address. There are a couple more machines on our LAN with the same issue. It seems any change made to the IP by the DHCP is not taking affect. As part of a possible resolution, we removed all instances of Jane_Doe that referenced the IP address the DHCP says it has, that didn’t make a difference, flushed the cache on my local machine and on the server, still didn’t make a difference. Any ideas?