How does search actually work?

I've noticed this a few times in the past, but never followed it up. Now I know for certain I'm loosing potential sales.

Current events in the news led me to check if some old clips of mine have had any increase in views.
A series of clips I've had on P5 for several years refuse to show up in searches. Despite using exact keywords, or even exact file names.
As an example, results show only 37 clips, maybe 8 that are relevant and none of my 21 or so that are key-worded and named exactly as my search terms. Change search terms a bit and maybe 1 clip will show up, but not the other 20, despite them having the same keywords.

Have I been doing things wrong for the past 11 years?

If it’s any help, by way of comparison, I’ve just searched for some of my own clips from years ago and they all showed up within the first few pages of the search results.

A friend recommended this site because he knew someone who used to sell on it, but now I am actually trying to use it I am getting the impression that is not being maintained and is not a serious site.