Complain all you want..

[Kirosuu]

That is the nature of RNG. However it may also be the nature of simply not knowing exactly what circumstances causes the items to drop. It is believed to be completely random, but this may not be the case. Perhaps you and your friends should share your experiences and see if you can come up with any findings.

from my experience, party will increase drop chance
6 from 7 atma i got is from party with less than half hour ~ 1 hour each