the metric "Memory|Demand|Provisioned (KB)" appears to have stopped collecting, there is data showing up to about a month ago however nothing since then. Other metrics appearing to be collection for the same object and I have confirmed that this particular metric is enabled under the policy. how can I troubleshoot..?
Is this for all objects or only some? have you verified the vCenter counters are visible in the performance tab?
Has vCenter been upgraded or the statistics level changed?
Check the policy associated with the object you are trying to get this metric from, and make sure that under Section 5, collect metrics and properties, that that specific metric is enabled (either locally or by inheritance) for the object level you are looking at. Many metrics are not enabled depending on the object level.