Every predator has their own tip page that explains how to obtain the willing scene. In game, open Help > Predator Tips - Spoilers, then find the one you're interested in (Lamia/Harpy).
For essence collection, you'll have to be more specific. It has been tested and works. If there is some issue or bug that is occurring, that would need to be shared. If there appears to be no issues, then you may need to say what you're trying.