I am at the quest objective and no Harpies are spawning. At first it was outside the cave and no it is in. Is it a bug? How do I load more harpies?

Edit: They finally spawned out of nowhere, got 4 nests and then went into cave to get other 3. Got 1 and then they disappeared again.

If you need more of a certain monster to spawn, either leave and come back later, or meditate 6-12 hours.