There could be a number of causes behind this -
1. Do have campaign qualification rules set up that'd allow people to flow through the campaigns only a set number of times in a given timeframe or only once?
2. Do you see the exact same activity for which you have set up the trigger (including the constraints value) in the activity log of people who didn't run through the campaign? Also, were they eligible for the send alert campaign SL filters when filled out the form/visited the webpage (i.e., when they performed an activity that could trigger off your send alert campaign)?
Let us know if you need our help in ensuring that the campaign's trigger matches with the activity for which the campaign did not fire. You can post the snapshot of the trigger campaign's SL and the activity from the person activity log.
3. Maybe a silly question, but do you see any send alert flow step/activity in the log? It could have been the case that the alert email may have landed up in the junk box/may have been quarantined. If you see a send alert flow step, but still didn't get an email then that'd mean that your campaign is A-okay.
4. Highly unlikely, but do you see the campaign queue stuck in processing the leads in the campaign queue?
If everything including the campaign trigger/filters/qualification rules line up so that the campaign should have triggered off, but for some reason, it didn't, then you may open up a support ticket to see if there's any issue in the backend.
Hope this is helpful! Please let us know if you have questions.
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