Ok, I should say "active" primarchs. Others are presumably lost/dead/sleeping ?
The Lion was taken by the watchers after the destruction of Caliban, he is currently in a form of stasis in the Rock but the Dark Angels are not aware of that.
The Khan chased Dark Eldar forces into a webway portal and has not been seen since, it was implied that he might be in the dark city.
Guilliman was placed in stasis on Macraage
Corax and Russ went on separate missions to the Eye of Terror
Vulkan vanished from the eyes of the Imperium, but he'd done that before
Of the remaining three, Sanguinius and Manus are dead and Dorn is dead depending on the source (or missing a hand and his armour and in self imposed exile for no reason.)