I just had this same issue, the alert was working just fine and then all of a sudden it stopped.
The issue was that I had added a recipient to the alert and typed their email incorrectly (without the @___.com). Once I fixed their email it worked like a charm.
Try sending to a gmail or other address outside of your work domain. If that works, you know it's a quarantine issue. Also, make sure the email alert headers are correct and complete (including an email subject). And as others have mentioned, triple check for typos on the email addresses.
We also had issues with alert deliverability. I set up a gmail address with the sole purpose of receiving alerts and test emails and forwarding them to my work address. That works every time (until I hit communication limits).
Yep, email alerts deliverability seems to be an issue even when white-listing Marketo. We also have some just not popping up for random reasons.