Given that this has happened on multiple accounts, it strongly suggests a new issue or a bug.
Here is a step-by-step plan that goes beyond the basic checks.
Step 1: The "Nuke and Pave" Test This is a simple but effective test to rule out any corruption in the existing setup.
Create a completely new phone number call asset.
Create a new conversion action specifically for this new call asset, and make sure it's set as primary.
Add the new asset to your campaign and let it run for 24-48 hours.
If this new asset and conversion action fire correctly, the problem is with your old setup.
Step 2: Use the Ads Preview and Diagnosis Tool While this tool is typically for ad serving issues, it can sometimes reveal tracking problems.
Go to Tools & Settings > Ad Preview and Diagnosis.
Enter your target keyword and location.
Look for your ad and see if the click-to-call extension is serving correctly. It won't tell you about conversion, but it can confirm the asset is live.
Step 3: Submit a Detailed Bug Report to Google Support This is the most direct way to get help for a likely bug. When you contact support, don't just say "my conversions aren't working." Be very specific.
Provide a clear timeline of when you first noticed the issue and on which accounts.
List the exact steps you have already taken (the four points you mentioned).
Include a screenshot of the Call Report showing calls and a screenshot of the Conversions dashboard showing a count of zero.
Mention that this has happened on multiple accounts, as this will signal that it is not a user error but a potential systemic issue.
The fact that you're an experienced user who has seen this on multiple accounts in a short time is a strong indicator of a new platform bug. The best use of your time is to submit that report and then use the workaround campaigns to maintain performance while you wait.