DHCP unable to obtain IP on large network

I am currently using the Digi Connect ME –C (NetOS 6.3) in an application for a client where the units are using dhcp to obtain IP addresses. So far everything has been working fairly well with one exception. When our clients try to add the units to a larger network (one with a subnet mask of rather then the units aren’t able to obtain an address. We have been able to use static IP addresses for the units in these cases but it is not the ideal solution. Does anyone know if there are some settings in the config files that need to be changed to allow for dhcp to work on larger networks? Or has anyone else had similar problems and knows of a solution?