Hi
If you definitely have an Alert rule then also, check that you don't already have an alert "somewhere" and the suppression settings. It might be that there is an alert from days ago which you haven't spotted where the repeat count is being incremented and no new alert generated.
Also, there is a water mark and the rule only checks new entries so if you are deleting lines then it might be that the rule isn't going back that far. This is explained here - https://marthijnvanrheenen.wordpress.com/2014/11/08/tips-and-tricks-for-monitoring-log-files-with-system-center-operations-manager/
Kevin Holman walks through this here - https://kevinholman.com/2009/06/19/using-a-generic-text-log-rule-to-monitor-an-ascii-text-file-even-when-the-file-is-a-unc-path/ - double check your steps against this (although it is an old article, it is still accurate today).