I have a feeling atma has a drop allowance as a lot of people seem to get atma within the first few fate in an area or end up grinding for hours and get nothing.

I'm at 3/12 atma now and all 3 came from NM fates and also within 5-20mins of entering that area. I've also spent hours in a single area and got nothing for it.
The strat i'm using now is to spend 30mins in an area and if nothing drops i change area. I'll see how it goes.