Can?t access Internet when using DHCP

We?ve been using an ADSL in our fixed IP address network to access Internet, everything was fine until we use DHCP service. The DHCP service is working cause we can connect to the network but we can?t access Internet in clients who use DHCP but we can in the clients with fixed IP. What can we try?? we?ve already defined the scopes between 2 servers (70% / 30%)and excluded the ADSL Modem IP address.