A Zero Conversions alert tells you when one of your conversion actions stops recording conversions while your ads are still running. It's often a tracking problem, like a broken tag, rather than a real drop in performance.
Which conversion actions are watched
Only the conversion actions selected in the ad account's Settings. Conversion actions you haven't selected there are ignored. See Choose your Conversion Actions.
When it's sent
Opteo checks each selected conversion action once a day. You get an alert when it has recorded zero conversions on each of the last four days, while the ad account was still getting impressions. Four days is long enough that a reporting delay is unlikely to explain it.
You won't get an alert when:
The conversion action normally gets very few conversions, so zero isn't unusual for it.
The ad account had zero impressions on any of those days, or got a Zero Impressions alert the same day. No impressions explains no conversions.
The same conversion action was alerted on recently.
What to do
The alert names the conversion action that stopped. Check its tracking setup first, for example the tag on your website or the conversion action's status in the ad platform.