The free-form search is optimized for exact title searches (also for creator names). Sounds like it's working as intended. And you can only use so many tags, to avoid abuse, but you can probably improve your tag selection. See how other people use tags, and which ones bring traffic.
But mostly, you shouldn't rely on itch.io searches to drive traffic. Be active in the community and on social media, and try other ways to make your game known.