In our case a host was marked as Unavailable due to SNMP items failing to report data back. However, when the host was restarted and the checks resumed, the host was not being marked as available again and as a result no data was collected until I restarted zabbix_server process.
Here is the relevant output from zabbix_server.log:
3778:20140227:011532.839 SNMP agent item "ifInErrors[port4]" on host "XXXX" failed: first network error, wait for 15 seconds
3783:20140227:011550.394 SNMP agent item "ifInErrors[port4]" on host "XXXX" failed: another network error, wait for 15 seconds
3783:20140227:011608.512 SNMP agent item "ifOutOctets[port1]" on host "XXXX" failed: another network error, wait for 15 seconds
3783:20140227:011626.635 temporarily disabling SNMP agent checks on host "XXXX": hos t unavailable
The UnavailablePeriod is the default 60 seconds, and it is my impression that the UnavailablePoller was not even attempting to check.