Had the issue recently where the DFM would stop reporting statistics on all of our volumes on some controllers. After spending an eternity digging around it seems that if you enable debug logging on the dfm you will see an error returned by the controller that it has run out of memory. You can increase it but please check this burt first http://support.netapp.com/NOW/cgi-bin/bol?Type=Detail&Display=472940
To increase the memory from the default of 262144 do “options perf.dblade_zapi.buf_limit and select a higher number (in bytes). Try 307200 (300k) first then slowly increase it if you have no joy.