If there's room in the boiler, add as much weight as you can. My 3rd Rail GS4 pulls all 14 GGD Daylight cars with out a problem on flat track. GGD cars heavy, and a 2-3% grade is probably going to eat up some traction tires.
Have you weighed the 2 engines to see what the difference is?
Did the traction tires get oil on them maybe, dust/grime making them slick?