No response from network element

This one has me stumped. I get this
No response from "MDR-IP20e" [10.72.0.155].161
 
but if I do a 
/usr/local/bin/snmpwalk -v2c -c xxxxx MDR-IP20e:161 ifDescr
I get a reply. The community is the same as in the nearest Defaults file
and works for other nodes (in a differant part of the network).
This network element is behind a firewall and the only thing I can think
of is that Cricket does something else to test connectivity 1st (but I
can't find any mention in the documentation).
 
Anyone got any ideas on what this could be?
 
Cheers
Wayne
 
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