Prowin Hop unit in VFC H&K 416

[Caldymoose]

Right so I need some help.

Setup: VFC H&K 416, lonex internals. Air nozzle and tappet plate are I think, the stock VFC ones, but its been a long time since I installed them.
Hop unit: Prowin (and yes, its been assembled correctly), Prometheus Purple rubber, Prometheus EG 6.03 steel barrel.

Removed the old Lonex hop unit (which worked fine with the same barrel and rubber) and installed the new Prowin one. The air nozzle just doesn't go into the hop unit as it should, resulting in all the air leaving via the BB feed neck, not the barrel. FPS is somewhere in the 50's, which is, well, not where it should be.

So, Ive taken the hop unit and barrel out of the upper receiver, and held it against the gearbox. And, what do you know, when I look down the feed neck, the air nozzle goes into the hop unit and over the feed neck as it should. So I know for a fact that the air nozzle can go into this hop unit as it should.

So whats changed between me holding it (tight, flush, barrel looks flat and all) against the gearbox and then when its installed? I can only think the hop unit is being held a fraction of a mm out of place, so how do I correct this? Has anybody else had issues with these Prowin units, or better yet, installed one in a VFC?