It's a little late, but I found that in my case, I was losing mouse capture to the containing scrollviewer almost immediately, so I canceled the event bubbling up using e.Handled = true; and that solved my problem.

 private void VolumeSlider_PreviewTouchDown(object sender, TouchEventArgs e)
 {            
     (sender as Slider).CaptureTouch(e.TouchDevice);
     e.Handled = true;
     return;            
 }