I came across an interesting issue that I thought should be shared. If you have a parent hostgroup with child hostgroups and one of those child hostgroups doesn't exist, all of the following child hostgroups won't apply their services to their respective hosts. I had a parent hostgroup with around 25 child host groups. About half way through the list, I had forgotten to remove a child hostgroup that was no longer defined. When running a sanity check, it said the hosts of child hostgroups abc didn't have any services associated with them. I eventually figured out that the issue was the undefined child hostgroup in my parent hostgroup_members list.