zabbix agent upgrade to 2.2.6 got many agent alert

[robertchen]

got many agent not unreachable status, but the agent ping is Up.
agent version is not upgraded 2.2.3

Trigger status: PROBLEM
Host Name: ***.***
Trigger: Zabbix agent on ***.*** is unreachable for 10 minutes
Trigger severity: Average
Trigger hostgroup: !VMs, P3

Item values:
Agent ping (***.***:agent.ping): Up (1)

Event date: 2014.09.12 01:00:00
Original event ID: 7297995

the agent is up, the alert is keeping sending out for 30 minutes escalation.