With the intention of revisiting some of the threads I have contributed to in the past I have input my TV user name santisuk into the search bar (top right) and clicked on all forums before hitting go.

This brings up 306 results today. Yet I have made 1,100 posts in the last 18 months and many of the posts I have made recently do not come up in search results.

Why is the search function so apparently randomly selective?

Try Google. It's historically done a better and more thorough job of indexing the forum than the board software.