not sure if this is actually a bug or if you are just unlucky. I played on the Taiwan servers years ago until they shutdown and even played on the DBOG private server and I never got that lucky. sometimes it took 3 or 4 hunts just to get 1 dragon ball to drop and other times 1 would just drop when I was just questing and not focusing on the hunt. that last dragon ball is probably just being stubborn. also fight some stronger mobs if you are lvl 34 it won't drop from a lvl 28 cause the enemy would be 6 levels lower then you. to get dragon ball drops the enemies have to only be within 5 levels lower or higher then you.