Cause: The toner cartridge may be damaged, or the lower fuser roller may be bad.
Solution: Some printers sometimes don’t pinch the paper tightly enough during fusing, causing smears. The fuser has a Teflon-coated hollow metal roller with a heat lamp inside, and another roller that pinches the paper against the hot roller during fusing. The lower roller is made of high-temperature silicone rubber. Those sometimes go bad as described above, but if you have replaced the fuser assembly recently, that isn’t the problem.
Look for any rollers between the pickup roller and the fuser, and clean them with naphtha or mineral spirits. If there is a black roller with a foam-like surface, it is electrically conductive, and is part of the mechanism that places electrical charge on the paper. Clean it with alcohol, taking care not to get fingerprints on it. Clean everything in the paper path: if there is a corona wire (instead of the conductive roller), clean it with a Q-tip dipped in alcohol. If this does not help, the toner cartridge might need to be replaced.