Make sure the valve is not pressing the tire bead out. With tire and tube mounted, half inflate the tire, push the valve up away from the rim. Make sure the bead it seated, then pull the valve back down into place. Inflate.

Also, make sure ther tube in not twisted in the tire. Before inserting, inflate inner tube just enough to retain shape. Insert tube, then install the remaining bead. A little talcum powder helps to keep the tire from twisting too. (a light dusting - don't go the "white lung" disease route)