Zenoss is not showing server memory graph suddenly

HI,

I am using zenoss core with SNMP Zenpack " ZenPacks.Nova.Windows.SNMPPerfMonitorSimple". I have added Microsoft Servers in Server/WIndows class and servers shown their CPU and Memory graph by using above SNMP zenpack but now sudden some servers failed to generate MEMORY graph and when i check these servers from memory data points an error will appear, Below is the link of the error. Kindly check and guide me about the same.

http://www.4shared.com/download/PXmMh_bNba/Zenoss_Memory_Not_Showing.jpglgfp=3000

Regards,

Arshad