Hi asheridan,
try adding this to /opt/zenoss/etc/zenping.conf:
data-length 8
*You can also alter this value in Settings > Daemons > Zenping config.
Please note that this works under zenoss core 4.2.4, prior versions needed to edit a *.py file manually.
I also been throught this scenario, they use nmap for pinging devices, and default ping data length sometimes is too big for particular devices. In my case a enviromental monitor was shown as offline, seems to be that some firmwares doesn't address "big pings".. 
Try this out, is you use an older version let me know which and we see which file you need to modify.
Regards,