Hy
Lem,did you try using the high flow on the pack (or eng bleed off) while using the Apu? If not working,then maybe it's time for the tech to do their work,clean some filters....
The ground air conditioning cart is connected directly to the mix manifold.So ,I wouldn't use both Apu and the cart on the same time.The cart has,usually ,a higher air pressure than the Apu,so,maybe a chance for a back press on the pack.This is the reason for the recomm to switch of the packs while using the cart.
The Ng has a more powerfull Apu,so you can use both packs with the Apu.No need for the cart on this case.
Brgds...
Alex