Hi
from memory 200 psi max in the boost valve (written on piggy back decal) If you run more you make the shock feel a bit crap and increase the likelyhood of a internal seal blowing, resulting in the need for a service or rebuild, which is pricy
In the main air chamber you can run up to 300psi
I'm guessing your blowing through the travel to easily.
Check your spring rate/air pressure firstly here
http://www.tftunedshox.com/info/spring_calculator.aspx
you should be getting a third sag sat on the saddle with propedal set to minimum.
If your heavier than 14/15 stone the dhx will work a lot better with a tune on the compression and rebound as there set for the joe average and if your above or below that range of weight they won't work so well.
You also fill the extra air volume air sleeve with grease to reduce the volume making the shock more progressive through it's stroke, making it harder to bottom out.