- The tire is defective (least likely).
- The tire is simply not settling onto the rim properly.
- You managed to twist/pinch the tube while installing.
If you twisted/pinched the tube then there will be low spots, where the tire does not seem to inflate fully. If you suspect this, of course, remove and reinstall the tube.
If you're convinced the tube is not twisted, slowly inflate the tire while you inspect the junction between tire and rim, all around on both sides. (There is generally a rib on the tire sidewall positioned such that it will be about 1/8" from the rim edge all around when the tire is perfectly centered.) With your hands, "work" the tire one direction or the other to keep this distance equal all around as you slowly inflate.
Sometimes tires are hard to get seated properly when new (because, eg, they were hanging on a hook in the shop for months), but quickly "take a set" and are no trouble if you have to dismount/remount the tire later.
I'll add that sometimes the problem is that the thickened portion of the tube near the valve has gotten caught between the tire bead and the rim. This will prevent the tire bead from seating on the rim hook properly in the vicinity of the valve. The solution is to completely deflate the tube, then press the valve most of the way in with your finger while you manipulate the tire in the area of the tube. This should cause the "captured" part of the tube to work loose.