Players being unable to pick up Specialization ammunition (Demolitionist)
I have the exact same issue until you hit the square to reload it. It reloads what is has in the clip and then allows you to pick up and load (1 at a time)
Only way to pick up from scratch is to totally unload the grenade launcher, Or as i have got used to doing now, reload when i have like 4 and 2 then it loads the chamber to 6 and 0 and allows a pickup of specialist ammo.
Ammo clip has to be empty for allowing pickup of ammo.
Hope this helps.