Like I've said, The more and more Mori gets into stuff, the more Gura thinks she's being wasted. It's obvious why she and Ame are leaving, because of massive imposter syndrome and COVER is to blame for continouously saying no to their project proposals.
To be perfectly clear, I am not blaming Mori, She deserves it for working really hard. But COVER inadvertedly demotivated other Holos because of their approach.
