Sounds like it has to be an air flow problem. Is there a way of setting your vent to concentrate the air flow directly at your tray? You are leaving your exhaust wide open? You might try directing a small fan at your inlet vents and set your tray as close to the vent opening as possible.
I don't own a tray but do have two of the 6 inch perforated tubes. I store my plastic bag of Traeger pellets in a damp barn with a dirt floor. Have never done anything to dry the pellets before filling the tubes with them. I concentrate my small propane torch on the pellets at the open end of the tube for 45 seconds, withdraw the torch, blow on the coals a couple times and stick the tube / tubes in my WSM with vents and exhaust wide open. Never a problem. They smoke for 3 & 1/2 to 4 hours every time.