It would help if you mentioned which ones, so we can figure out which have a main drop and which don't.
Turul, Fistule, Usurper, Lacovie, Ulhuadshi, Sobek, Cirein-Croin
Ovni
Both +2s without any trigger or TH, on three occasions
SE does not make things drop "randomly" I DON'T BELIEVE YOU!
Anyone having a glitch where ulegrande range doesn't show in list of teleportable zones for abyssea?
I did 50 Glavoid Shells right after the update and saw only stones. Didn't grellow any of them. I, too, have seen some mobs drop both +2 items without grellow however. It makes me wonder if some mobs need grellow to drop their "sub" item and some mobs don't have a "main/sub" just two equal chance +2 items.
I know this isn't a huge amount of kills and it could just be luck, but th8+ and grellow on 2 Durinn's and 1 Briareus all dropped both of their new +2.