I would agree with everyone saying that you should 1st work out how long a print needs to stay in the developer to ensure full blacks. I would take a small piece of paper (say 4" * 5") and expose under the enlarger (covering half up with a piece of card) for something like 30secs with the lens wide open, this should ensure enough exposure for solid black and hopefully still vivid white on the unexposed portion. Perform some tests as described by the other posters to ensure your papers fully developed and not fogged (either by the safelight or in the packet, which can happen if it's real old or someones accidently opened it). Another telltale sign that you're fishing the prints out of the developer too early is 'tong' imprints on the print.(posted 9211 days ago)Something, no-one has mentioned is the contrast of your negatives. If their muddy (ie. no contrast) then it will be difficult to get whites and blacks on the print at the same time. You'll either have whites with greys or greys with blacks. As Alan(?) said, what's the details on your paper? Are you printing on grade 5 and this is still happening or are you printing on grade 2?
Nige.