The common scenario's where alarm could not be acknowledged are;
-> User doesn't’ belong to the Operators group.
-> AlarmCondmodule is already acknowledged.
-> Bad time synchronization, future time in controller.
-> Different settings in Alarm Manager service - special configuration (Eg:Auto Acknowledge)
Applog and OPC AE server log files also gives some hints on what could be the reason.
As well as Arihant's suggestions, a common cause of trouble on older systems is that the alarm collector and alarm manager redundant services bcome unsynchronised.
Restart each Alarm Manager and Alarm Collector service one at a time and wait for the service to synchronize before restarting the next. Sometimes it is also necessary to stop/start the complete alarm manager service group, but try to avoid this if possible.