VMWare java SDK: When doesn't an available PerfMetricID report Data?

Question

I'm trying to use vmware sdk for java to collect the perfomance data of each entity (cluster/datastore/Host/VM) in the vmware environment. The idea is to get the available PerfMetricIds for the target entity with queryAvailablePerfMetric, query those and report the details of the counter, the timestamp and the value.

However when I get the PerfMetricIds for an entity, not every detected (returned) PerfMetricId is reporting data. For example for each Datastore I get at least 4 ids which do not return data when queried, these IDs represent the counters associated with the average number of read and write operations, and for a cluster I'm missing the cpu usage, and so on ...

so I was wondering when does this happen? Shouldn't every metric returned by queryAvailablePerfMetric report data? what am I missing here?

Minimal code snippet:

// VMWare credentials
String vmwareUrl = args[0];
String vmwareUsername = args[1];
String vmwarePassword = args[2];

// connect to vCenter
ServiceInstance si = new ServiceInstance(new URL(vmwareUrl), vmwareUsername, vmwarePassword, true);
// get performance manager
PerformanceManager perfMgr = si.getPerformanceManager();

// define the time window (the last one hour)
Calendar calTo = Calendar.getInstance();
Calendar calFrom = Calendar.getInstance();
calFrom.setTime(calTo.getTime());
calFrom.add(Calendar.HOUR, -1);

// get any datastore for testing purposes
Folder rootFolder = si.getRootFolder();
ManagedEntity[] datastores = new InventoryNavigator(rootFolder).searchManagedEntities("Datastore");
ManagedEntity me = datastores[1];

// query all available metrics for the entity
PerfMetricId[] availablePmis = perfMgr.queryAvailablePerfMetric(me, calFrom, calTo, perfMgr.getHistoricalInterval()[0].getSamplingPeriod());

// create PerfQuerySpec
PerfQuerySpec qSpec = new PerfQuerySpec();
qSpec.setEntity(me.getMOR());
qSpec.setMetricId(availablePmis);
qSpec.setFormat("csv");
qSpec.setStartTime(calFrom);
qSpec.setEndTime(calTo);

// query perf
PerfEntityMetricBase[] perfValues = perfMgr.queryPerf(new PerfQuerySpec[]{qSpec});

// Printing
System.out.println("Found pmis (CounterIDs only): ");
for (PerfMetricId pmi : availablePmis){
    System.out.print(pmi.getCounterId() + ", ");
}
System.out.print("\nPmis with values:");
int pmisCount=0;
for (PerfEntityMetricBase value : perfValues) {
    PerfMetricSeriesCSV[] csvValues = ((PerfEntityMetricCSV) value).getValue();
    pmisCount += csvValues.length;;
    for (PerfMetricSeriesCSV csv : csvValues) {
        System.out.println("Counter ID: " + csv.getId().getCounterId() + " ---- Metric instance: " + csv.getId().getInstance());
        System.out.println("\tInfo: " + ((PerfEntityMetricCSV) value).getSampleInfoCSV());
        System.out.println("\tValues: " + csv.getValue());
    }
}
System.out.println("---------------");
System.out.println("Detected PMIs: " + availablePmis.length);
System.out.println("PMIs with values: " + pmisCount);

Any help (or discussions) would be appreciated