Am I doing something wrong with Atma farming? I have not dropped a single item.

[HEC]

Yes - you're doing it "wrong". Atmas were never meant to be farmed or hunted for in atma parties or endless whole day FATE sessions - the desing is to collect them "randomly" as you happend to pass active FATE on your way to do something else. I've only recently started the Atma quest, got my first one just after two FATEs which I've joined only because they were active just right next to me. And I'll not "chase" or farm those so I might get it in about the same time as you but in effect spending about twenty times less griding actual boring FATEs and doing something else I do enjoy ...