After the pumpkin has set and is around volleyball size, you will need to reposition it.  This takes time and patience moving your pumpkin.  You will want to move the pumpkin in the evenings when the vines are warmer and more flexible.  Move your pumpkin only a little bit each day.  It can take up to a week moving your pumpkin a little each day to get the stem 90 degrees to the vine.