I assume that you've got fluid in the unit. I usually put 20 drops in my GP and it could probably accept more.
You can pry off the plastic top section of roof that has the two smoke stacks.
That then exposes the internal black plastic funnel and smoke unit beneath. The foam gasket that is glued to the underside of the removable roof tends to come "unglued" due to the oily smoke fluid. It may have slid around and shifted and is actually blocking the stacks. I cut a fresh one from some black craft foam sheets from Hobby Lobby.
While you have it open, you can look inside the smoke unit itself as SF158 mentioned to see if the batting is charred.
I had a loco into Lionel for repair and it was apparently left on the test track without enough fluid and it came back with badly charred batting.
Also, blow down the unit to make sure that an air bubble hasn't formed in the unit or the funnel. It happens and can cut smoke off completely.