From a well exposed and processed 35mm frame, you can make a wonderful 9x12 print.
If you insert a #2 filter in your enlarger and make a contact print, then make a enlargement with that same setup, the contrast will appear lower on the latter. I don't know why but it always happens to me. Maybe it's a visual thing... or maybe it's real. I only use contact sheet as a guide, and work on making a great print. THAT is a separate process.
But, there's also a matter of degree... if it is excessively lacking contrast, sharpness, or just plain muddy, then you'll need to inspect your lens closely. It is not unusual for enlarger lens (which we almost always get them second hand) to have some light haze. More than half of my lenses came this way. If this is the case, you'll need to clean that.