So, I'm in the process of troubleshooting this...but here goes.
I have a pretty huge music library. It's probably > 1TB and > 130,000 songs. I have noticed that, even though it is indexed by the plex server, songs do not show up in the search. For instance: I type "tom petty down" in the search and get no results. If I type "tom petty" I will get both "Tom Petty" and "Tom Petty & The Heart Breakers". If I click on "Tom Petty" and drill down into "Full Moon Fever" I can play "Runnin' Down a Dream" no problem.
No combination of "tom petty" and "down" "runnin" "dream" or anything like that will return the song.
Now, what I suspect might be happening is that maybe the library is not fully indexed? Or there is an index limit?
If I find that it is not fully indexed (and even if this is NOT the case I still think that this would be a good feature to add...) could you implement a way to scan specific libraries for new content on an interval of say 15 mins (where I add TV shows) and others at an interval of several days (say where I add music or movies or pictures).