This may happen due to various reasons, in particular, due to incorrect settings of the vCenter Server statistics interval. Check for the following error in the Collector log:
EXCOL+** [VP160] queryMetrics failed, A specified parameter was not correct. querySpec.interval at nworksCore.Collectors.VimClient40.queryPerf(PerfQuerySpec qSpecs)
The log can also include other errors:
VP050 Error encountered retrieving performance metrics for a cluster on ql1vcenter1. The VMware API error returned was 'A specified parameter was not correct.querySpec.interval'.
VP210 Datacomm connection lost to VC target ql1vcenter1. The VMware API error returned was 'A specified parameter was not correct.
querySpec.interval'. No performance data will be obtained from this source until connection can be restored.