Modded enchantments, such as those from Ensorcellation, can't be put onto the neptunium bow. I asked the COFH folks about it:

Phyne_: It seems that the bow enchantments from Ensorcellation cant be put on
modded bows (aquacultures neptunium bow). If this is a problem with that
bow specifically, I'll head over to their issue tracker instead, I'm just not sure
myself.

King Lemming: Intended. Bows need to add a capability for it.
King Lemming: I might be able to invert that behavior...will look into it

Figured I'd put this here in hopes that it's an easy addition for you. If not I guess there might be a fix coming from their side.

Since the mod in mention is closed source, I would have no clue what capability that is being referred to.

Well I'm sorry for linking to essentially the wrong place, but COFH isn't actually closed source. Apparently this is the place to look: https://github.com/KingLemming/1.14/blob/master/CoFHCore/src/main/java/cofh/lib/capability/CapabilityArchery.java