VPN set up correctly but sometimes I don

Someone help me please!!!! I have a VPN connection set up on several computers all over the country. I have two of these computers that can’t seem to get an IP address. I can get IP addresses for the other computers that connect, but not my problem children. They have both worked before and I’m using a Shiva VPN box to connect to. I do get the rest of the DHCP info, just not the address. HELP