You may have noticed that when you go to set custom host reserve properties in a host group these setting do not automatically propagate down to the individual hosts with those groups:
When you set the properties for “Host Reserves” at the Host Group level in System Center Virtual Machine Manager 2008 or SCVMM 2008 R2, you are, in essence, setting template values for host reserves for newly added hosts only. A common misconception is that when you set these reserves on host groups with active hosts that these reserve values will propagate down automatically or that these hosts will inherit those settings. That is not the case.
You can get these settings to pass down by leveraging the Host Group in the form of a template. All newly added hosts that are placed directly in a host group with custom reserve settings will inherit these settings; however, this is the only way this occurs.
If the host are already added, then the only way to get these settings to take effect on existing hosts is to remove the host and then re-add it directly to the host group (with reassociation.) Simply moving a new host to a host group after adding it as a host will not suffice.
Note: This information was originally provided by Steve Thomas, Senior Support Escalation Engineer, on the System Center Virtual Machine Manager Team blog: