Flash Button Behaviour won't go to previous frame

Hi, I'm in desperate need of help. I have a flash gallery for a client which is proving to be a pain in the butt. I have 2 buttons. Obviously one going forward and one going backwards. The forward button is working on a behaviour to skip to the next tween frame. Yep the back button which I've used to the same script from the forward button, just thinking I could change it to Tween.preFrame, it will go forward, but not back (as the back button)

I can, at times get it to go backwards per frame, but I need it to go back to previous Tween frame.
Revatively new to flash so some help would be a major help.

this is the script I have on the back button so far.

on (rollOver) {
play();
}
on (release) {
gotoAndPlay.Tween.prevFrame();
//Movieclip GotoAndPlay Behavior
this.gotoAndPlay("15");
//End Behavior
}




(this is the one for the forward button
on (rollOver) {
play();
}
on (release) {
_root.main.Tween.prevFrame();
//Movieclip GotoAndPlay Behavior
this.gotoAndPlay("15");
//End Behavior
})

and if anyone could help, can you tell me why I can't make it go forward on release not rollover. when I change it to
on (release) {
play();

it doesn't move.


thanks. hope this is easy to understand. Im desperate.
thanks