You "checked the spring for tension." The spring, like the rest of your loader, is probably over 60 years old. It might not provide the tension it did in 1955...
Try the light finger pressure right at the center of the scoop (not off to the side) and see if that also helps move the coal evenly toward the conveyor. If it does, you might try a new spring. Part 397-23.
Another thought; the bottoms of the cam followers can get gummed up with a coating of oxidation and crud from the base plate. Try a gentle cleaning on the bottom of both cam followers, as well as the surface of the base plate underneath.
