A common cause of that is when your tube slips past the wire bead of the tire and forms a bubble. The wheel can be ridden on it like this, but it will eventually explode. Make sure the tube doesn't get between the tire and rim. Once the tire is mounted you can "roll" the tire away from the rim on both sides with your fingers to peak in between the rim and tire and make sure it isn't. That or your tire isn't seated correctly.