Unable to receive boss atlas skill books

I pretty new to the game so I'm not sure if this is a bug or if I'm doing something wrong. I received the first two books (lvl79andlvl80) on the same boss kill and after that I've tried to get more. I've used irradiated tablet on +1 monster level node creating level 81 zone, killed the boss but the book won't drop. I've done this multiple times on corrupted and cleansed zones. I've empowered the boss too.

I've used all the points too. I've also done the Arbiter quest line. Didn't work before Arbiter and Isn't working after either.