DHCP lease renewal issue

I have a issue with some clients not beig able to renew their lease after it expires over the weekend. This happens on Win95 & Win NT workstations. I can renew their lease on the workstation by using winipcfg and ipconfig respectively and then having them log off and back on. I don’t see any error messages in the event log for the PBD or BDC. I believe this to be a server related issue but I could be wrong. Any advise would be great.