The ball could be going backwards for 2 reasons:
1. you are not bending your legs or
2. you are swinging your arms too high
When you bump the ball, you want to do this:
-Stand square in front of the ball (if you can)
-Bend your legs at about a 100 degree angle
-Extend your arms as far as the can in the bump position making sure the ball will hit your Forearms at a flat place (not your hands)
-When the ball hits, don't swing your arms! just lift up n them a little bit and unbend your legs.
-Remember to face your arms and body in the direction of where you want the ball to go
Good luck!
Correct bump position picture:
http://www.juneauempire.com/images/110305/11577_500.jpg
http://rich410898.files.wordpress.com/2008/10/volleyball-dig2.jpg