The most common reason none of the members are being updated when you are editing your Master Service within a Service Group, may simply be because you are working with the wrong Master Monitor.
When a Service Monitor is used as a Master in a Service Group, it is copied based on the original monitor so changes to the original have no impact on the Service Group.
To ensure that you are working with the correct master monitor, follow these steps:
You should now see a page listing the master monitors for that service group and a list of all elements that have member monitors within this group. To edit the master, click the Edit icon next to the master monitor name.
For more details please review the online documentation - Using Service Monitors