It's literally about luck. I got my very 1st Atma (Crab) on the 1st FATE I tried. I was averaging 1 Atma a day while having only 1 hour of grinding. After the 4th day I was not getting any. Then again I got 1 random one every other day. 2 days ago I got 2 Atmas of Scorpion on back to back FATES. I'm currently at 7 or 8 Atmas plus my extra Scorpion Atma. Personally I don't mind Atma farming. It gives something to do while out in the field or waiting on a Duty.