I do not know what is considered and weighted to sort games into manual indexing vs. automatic indexing. The manual indexing queue is just so big and staff is so few. When they wrote the docs about indexing, a few days was correct.
The thing is, Itch is overrun by bad actors. Even with all their filtering and automatic, if you look closely, you can see all the things they still miss. Imagine how many things they do block by that manual indexing thing. It is rather rare that they intentionally not index a game for violating the quality guidelines - that's what the guidelines imply at least. But it often takes many weeks these days.
Because of those bad actors, it is also unlikely that they will implement accurate feedback why something was not indexed right away, or even if it is indexed. They could just as well give a blueprint on how to avoid manual review by staff.
Temporarily being delisted till staff looks at a game can even happen for an already indexed game. It happens even to big and established developers. So 7 out of 8 indexed right away does sound quite good. Just to give a perspective, I did see several hundreds of hacked accounts posting malware. They cannot give indexing protection to an account, just because that account has posted games in the past or is years old.