Sometimes you can't activate this even though you have the necessary tributes in your field / and or graveyard. For example activating grass is greener on first turn, or after doing a synchro summon with two zombies and those 2 zombies in the graveyard. Yet for some reason on the next turn you can!?!!?!?
It's not a Bug. It's a feature
probably because that one shiranui card that special summons specifically states you can only special summon ZOMBIE monsters that turn and not DRAGON monsters