Due to a limitation on the maximum allowed performance counters that can be retrieved in a single call, polling does not work correctly on large vCenters.
This issue is caused by a limitation defined by VMware.
In vSphere 5.5 Update 2 and later, and in vSphere 6.0, the number of entities that are included in a database query is limited. By default, VMware enables the collection of 64 performance counters in a single database query.
For reference, Virtualization Manager collects three performance counters for every VM, and five performance counters for every host.
For more information about solving this issue, see the VMware Knowledge Base.