Lets see if we can compile a list of where it takes longer to get atma to drop. Personal experience so far was:

Southern Thanalan: Atma of the Scorpion
Eastern Thanalan: Atma of the Bull
Western Thanalan: Atma of the Twins: 6 hours no drop
Central Thanalan: Atma of the Scales

Central Shroud: Atma of the Maiden
East Shroud: Atma of the Goat
North Shroud: Atma of the Archer

Upper La Nocea: Atma of the Water Bearer: total of 4 hours 30 minutes no drop
Middle La Nocea: Atma of the Ram: 1 hour
Outer La Nocea: Atma of the Lion: 2 hours
Lower La Nocea: Atma of the Fish:2 hours
Western La Nocea: Atma of the Crab:3 hours