All the Prison of Elders bosses besides Skolas will not give me their grimoire cards regardless of how many times I have killed them. The House of Wolves missions: Gone to Ground and House of Wolves do not pop for me no matter how many times I beat them. Regarding the Deathsingers grimoire you just need to kill them on the mission where you have to collect Crota's essence, the stealth mission on Crotas End.