In Ptolemy's earth-centered model for the solar system, Venus's phase is never full as viewed from earth because it always lies between earth and the sun. In reality, as Galileo first recognized, Venus is full.
Venus is closer to the Sun than Earth. That is why we cannot see the full Venus. However, Venus is full (whenever it is on the opposite side of the Sun from Earth). From Earth, Venus's phases range only between new and crescent.