Check if the alert is enabled or not
1. Login into Alert Manager.
2. Navigate to Define/Alert.
3. Query up Alert in question.
4. Check Enabled box.
Also check if workflow mailer is up and running?
Refer How To Send An Email In A Simple Periodic Or Event Alert? (Doc ID 1162153.1)
Ensure if you are able to send test mail through workflow mailer ?
You may check if the messages are getting queued in mailer queue with below query
select NVL(substr(wfe.corrid,1,50),'NULL - No Value') corrid, decode(wfe.state,0,'0 = Ready',1,'1 = Delayed',2,'2 = Retained',3,'3 = Exception',to_char(substr(wfe.state,1,12))) State, count(*) COUNT from applsys.wf_notification_out wfe group by wfe.corrid, wfe.state;
For one such notification for which the email notification is not sent, please run $FND_TOP/sql/wfmlrdbg.sql and upload the html output.
$ cd $FND_TOP/sql
$ sqplus apps/<apps pwd>