Being you do not use it to often , and may have not gotten it 100% clean on the last cleaning , I would start with a tear down of the brush and inspect the O-rings,
By the looks of the parts list for master airbrushes it is all rubber o-rings in them, Heat and cold can effect rubber making them hard and crack, Dirt in the air valve can also play a part in air not coming out the nozzle.
But I have had this happen once before , It ended up being the air tube(when you take off the nozzle you will see a small hole where the air flow through the brush) was clogged with dirt that must have been picked up while sitting . I took a thin piece of copper wire and passed through the tube until I had good air flow from it.
Also check the cap its self , it may have something in it the is blocking the air flow.
Hope this helps.
Sorry for being harsh but , It makes it a lot easier to help when you are given as much information as possible.