Air Suspension Problem

I have a 99 W900L with the AG 400 suspension hooked to a 55 ton Talbert lowboy trailer, sometimes after loading the trailer and hooking back up to the trailer the suspension won't air up.
Just yesterday it took 10 minutes before it aired up after raising and lowering the trailers gooseneck and moving the truck/trailer and charging the brakes to make the compressor run the air up to 120psi.
I also noticed that sometimes going down the road loaded the tractor feels like the suspension isn't fully inflated "truck leans" or feels like it is leaning.
I always drain the air system at the end of the day and the air dryer filter has been replaced, i purchased a new leveling valve but i didn't want to replace it until i posted this problem.
Keep in mind this is my first air ride tractor, i have only had Mack trucks with camel back suspension before this.
Any ideas would be greatly appreciated.

 

Yogibear, thank you, i looked up a thread about adjustment settings on the ag 400, very informative!

GrapeApe, thank you, didn't know that the trailer had a valve that did that.
The dump valve in the tractor works fine as far as dumping the air, when i first started using this tractor i would dump the air in the tractor before disconecting the trailer from the gooseneck and air it back up as i pulled away from the trailer so the eyes on the gooseneck have ground clearance, never had to wait for the tractors suspension to air up, dumped or not, its when the trailer is reconnected to the gooseneck with weight on it the suspension doesn't compensate for the weight and the air bags flatten out.