My cases extract hard from my revolver even with a mild load. What causes this?

First, clean each chamber thoroughly. It could be erosion or residue from shooting short cases and then switching to longer cases. An example would be shooting .38 specials in a .357 Magnum and then shooting some .357s. The .38 leaves a ring of residue at the case mouth. The .357 case is enough longer that expansion from firing along with the residue under the mouth of the case make extraction hard. After you have cleaned each chamber, check for erosion, tooling marks or rough spots. They are another cause of hard extraction. If that is the problem, the firearm will need to be sent to the factory or a gunsmith to be polished out.