DHCP and IP release on XP Pro

I have a problem with 2 laptops with a built in wireless card running XP Pro. They wouldn?t grab a IP number from the DHCP server. Using a network cable everything works OK. Using the wireless card the machines always assign an automatic IP and will not release it using ipconfig release & renew command. Windows XP Pro software sees the router signal and reports it is connected but has no access to the network and has wrong IP number. Tried different laptop with wireless card and it works fine. What am I doing wrong?