There are three different techniques I often use to make smoothly scrolling backgrounds/floors on a loop, depending upon what the project requires. Assuming right-to-left movement:
1) Use two instances of the same sprite the width of the screen and wrap them around each other
2) Use one instance double the width of the screen
3) Use a tiled background one tile larger than the width of the screen
I've demo'd each here:
The main thing is to reposition with respect to the current X, otherwise you can start to get seams and stutters in the scrolling.
Hope that helps