I just got back to the experiment.

Light: a bank of 8 30W HITACHI tubes about 4.5 inches above the print (the tubes are newly changed)
Paper: Bergger COT-320
a 21 steps tablet(I couldn't get the 31 steps tablet locally),half of which covered by Pictorico Ultra Premium OHP Transparency Film

I exposed for 20 minutes and 40 minutes respectively, but the steps are all far from black,yes,even the part not covered by OHP film. Then I changed the glass from 5mm to 2mm and exposed for 40 minutes.Same result.

Frankly speaking, it's making me feel despair. How come you can make it in just few minutes? I have no idea what I've done wrong. What should I do? To expose for two hours is not a good idea,right? I understand too long exposure is not right for making palladium prints.