DNS and DHCP running on Netware servers NW 6.5.5 eDir

DHCP is dynamically adding hosts and addreses to DNS but not always
removing the old address when the host gets a different address from DHCP.
When this happens the address returned to a dns query is the first listed
but that is the old one which has since been allocated to a different host.

This upsets the support techs because they cannot get to the correct
machine with remote control software if they try by machine name.

Any ideas what might cause this? or what i can do to prevent it in future?

Is there anything I can tune to improve the sync between DHCP and netware
Dynamic DNS?