I'm guessing that Specna Arms B06 (I know, M16, but same thing internally)
Could be any number of things. Hop rubber lips protruding too far, a worn tappet plate, a slightly miscast hop unit, a gypsy curse.
If in doubt, pull the barrel and re-seat it in the hop, and check if BBs will drop down the feed tube without sticking before re-assembly.
Something else I've have luck with is popping a 6mm drill bit up the feed tube and verrrry carefully creating a tiny indent in the top of the chamber. We're talking light pressure and a few turns, or even go in reverse: you're just creating the slightest of concave shapes to give the BB something to key into.