Are you using a dynamic GUP list of the single GUP settings?
Are the clients all RU5 or above?
Below RU5 the you can only use a single GUP per group.
RU5 and above added support for dynamic GUP's based on computer name wildcards, IP address, registry key, Operating systems, etc.
On RU5 and above a SEP client checks its own IP address and subnet mask and will only get updates from GUP's that fall in the same range.