+1 to what Martin says above.
I'll add that you should make a test strip to determine print exposure and that you should use the print highlights as your benchmark. In other words, find the enlarging time that gives you the highlight values you want. Adjust contrast to get the blacks you want.
If your enlarging times are too short (and a thin neg may be the culprit here, especially if you need a rather high contrast grade), then stop down or use neutral-density filters to get a more manageable time.
If you have a color head, you can dial in neutral density by using equal amounts of yellow and magenta (cyan isn't needed for black-and-white printing). This will give you longer times.
Even if your negative has shadow detail, it may be "thin." This could be due to underdevelopment or a low-contrast subject. In either case you would need a higher contrast grade than normal for the print. If this is consistently your problem, then increase your film development times.
Underexposure will also result in a thin neg, but the shadow values are not recorded. This results in less overall contrast and prints with gray shadows (not black) when printed at normal contrast and inky black shadows and heightened contrast for the mids and highs when printed at a contrast grade that gives a full range of white to black.
Hope this helps,