When you fill a pps cup and put it on the gun, the first thing you are supposed to do is turn the gun upside down and fully pull the trigger until all the air is out of the bag and the gun starts to shoot paint.
I have never had a problem with the lph 400 using the pps system.
The only thing that comes to mind is you have very low air pressure at the gun or very low air flow at the gun or both?
There needs to be enough suction created (low pressure area) across the guns venturi to pull the paint out of the cup when inverted.
If it will pull the air out and then paint when first filled as I mention above there is no reason for it to know work.
I have never had my pps cup bulge or indicate any pressure being introduced into it.
I do not think a properly working gravity gun will pressurize the cup, if it did then when using a standard cup there would be air bubbles appearing in the paint in the cup as the lid on the standard cup is vented. If that vent gets plugged the gun wont spray properly.