aperture type-ahead in the Caption field

I'm identifying photos added over the course of several field trips and trying to figure out exactly how aperture's 'type ahead' feature works?

Specifically, all of the photos are in the same Project. The field in question is Caption. When I start some text I'd like Aperture to offer earlier field contents but ht performance seems a little spotty- sometimes working, sometimes not. Very possibly some user error as the names are scientific animus of plants like "Philodendron selloum | Selloum".

These are obviously very easy to mis the spelling hence the need fort type ahead but I don't see the underlying rules?