ESX monitor

I used java program for esx monitoring (wrote in VMware VI and vSphere SDK by STEVE) to get the performance value for CPU, memory, disk I/o, bandwidth , it work fine and i get correct values, bur some time i got (-1) instead of any value,.

the part of program to get performance values

metric.setCounterId(98);

    metric.setInstance("*");

    PerfMetricId metric1= new PerfMetricId();

    metric1.setCounterId(6);

    metric1.setInstance("*");

    PerfMetricId metric2= new PerfMetricId();

    metric2.setCounterId(143);

    metric2.setInstance("*");

    PerfMetricId metric3= new PerfMetricId();

    metric3.setCounterId(125);

    metric3.setInstance("*");

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what is the wrong?

any suggestion?