Hi Jennifer - adding more levels may be best best achieved by adding more layers and/or more flying text boxes, rather than adding a new slide.
There are a lot of variables that depend on the position of the slider, and duplicating the game slide will create a second slider and a new slider variable - breaking the connections established in the main game.
This demo is now quite old and I too have noticed that Storyline no longer appears to support GIFs as slider thumbs, which may explain why the bird no longer moves.
You may find the blog I wrote at the time helpful?
https://www.linkedin.com/pulse/flip-bird-jonathan-hill-assoc-cipd/ In the meantime I am working on a updated version of this demo using the latest version of Storyline. Watch this space.