There are two likely scenarios there. The first one is that the airbag is leaking. I just replaced some airbags in the rear of one of our Phantoms last week. It's pretty obvious if that's the case though, because you would have said you heard the air leaking. So I'm leaning towards possibility number two.
It's possible that the leveling valve for that side is either broken or out of adjustment. The leveling valve will look like a valve with an arm on it, to which air hoses are connected. That valve will be mounted on the underside of the floor, at least I think so for the front. From that leveling valve is a rod about 3/16 of an inch in diameter that drops down to the axle. Since you say the airbag is partially inflated but not raising up the bus, my guess is that the leveling valve or the drop rod is not sending sufficient air to the airbag. You could try disconnecting the rod and then manipulating the lever either up or down to see if that changes the height on that side. If you push the lever arm on the valve up it should add air to the airbag and raise that side.
We have two Phantoms and I'm pretty familiar with them. Those are the two most likely scenarios. If it's something else, it's going to be more obscure.
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