Have you printed these negatives before and had satisfactory results? If so, it could be old paper or fog. Otherwise there are a couple of possibilities that haven't been mentioned. The first question would be whether the negative has enough contrast. A surprising number of people underdevelop their negatives. (They also often underexpose them, which creates an impossible situation.) To some extent you can correct this my printing at a high contrast grade, but there are limits. Another possibility is that the print is not being developed sufficiently. Prints need to be developed pretty much to completion - between 90 and 120 seconds for most developers at standard dilution.