I remember playing some classical when i was 12,or so, and mumsy always said to drop your shoulder and bring your thumb round to about the mid point of the neck - or further if you need to- use the thumb as a kind of pivot or "anchor" at the back for your fingers to get to where they need to on the fretboard....have a look in a classical book for the picture of the correct positions as they do say that position is v.important in classical guitar.