Steps to reproduce:
I had added two oracle instances on the same machine for monitoring. And they have their own alert log respectively. Corresponding items of type 'log' and triggers were added for them. The key of the item is "log[{#ALERTLOG},"ORA-|Checkpoint not complete|Can not allocate log|fail|halt|abort|panic",,,skip,]".

Result:
I finally found that only one of these two triggers remained valid as when there was an ORA error rise in the alert log of the second instance, the second trigger was not triggered. And the query
"select lastlogsize from items where hostid=xxx and name='alert log'"
in zabbix database returns zero.
As in the screenshot below, hostid 14321 and 14322 are two Oracle instances on the same server.

I appreciate your help and replies. Thanks.