Both show four messages for the past month where there should be hundreds.

Sorry there is no obvious answer and still not seeing similar reports so far. Where is this happening.

In my forum, Believers, Unbelievers,  Ex-Believers.

Are you using Classic or Zeta? I got loads of messages for you using search with Zeta. 
 

I celebrated too early, search option (using Zeta) is no longer working (again).

https://forums.delphiforums.com/allthingsirish

I set it to zeta once and chose classic display.

I see messages too now and then, but then they are gone again for days at a time.

At all? Searching for your own or a specific member's posts?

Same as I asked Meddly are these any searches or specific ones?

Specific members...also, from Anyone to Anyone, from three months to today or from yesterday to today.

gunter managed to fix it when I complained but today it went back to "not working" at all.

Search seems completely out there. Zeta, Classic, specifics, non specifics... searching for keywords... nada