What Slipin is suggesting is a good way to tell if any of your frets are messed up and causing the problem. If every note sounds clear individually, they should all sound clear when played at the same time. When learning a chord, start by placing one finger at a time until it rings clear, then add another note and continue until the full chord is ringing. If you reach a note that sounds bad, look around and see if one of your other fingers have slipped and is not muting that string.
Also try shifting your fingers around and make sure you are playing right up against the fret. On top of the fret and too far back will make it sound dead.
