Something else to check, especially if the drivers are not sprung, is whether the drivers all contact the rail evenly.
As already mentioned: add more weight, replace traction tires if they seem hardened, clean rails.
When space is limited, it may be possible to reshape pieces of lead to fit the available spaces. I've hammered old .22 bullets into small shapes to fit in HO locos. Lead fishing weights of different shapes and sizes can be adapted for O gauge locos.
Something else that might help: occasionally I run a home-made rail cleaner car around the layout with very fine grit sandpaper to remove light oxidation on the rails. I've noticed on my HO layout that this may help traction somewhat by removing the glaze on the rails.
