The exact mechanics of Snatch are unknown. There is data which suggests that it does work... ... probably...

Anyway, Snatch stacks. You went from 3 -> 4 -> 5 -> 5. The maximum stacks is 9. Skuld simply casts Snatch1. You can check debuffs (such as Snatch stacks) by clicking on the mob name when attacks are not taking place.

It is unknown how Eidolons with drop rate increase work, and I don't think anyone has even started to gather data on does it work or not. Probably does.