I have never heard of this before but perf counters collected by Harvest that have a value of 0 are not forwarded to Graphite. Could it be that these systems are idle and simply haven't had any read/write traffic yet? An easy test is to just make some read/write traffic and see if the metrics appear. Note that in the Graphite native interface you have to to do a F5 browser refresh for the tree to be updated to display new metrics files.
Cheers, Chris Madden
Storage Architect, NetApp EMEA (and author of Harvest)