Thank you for your feedback. You didn't do anything wrong. The game doesn't spawn any new asteroids when the player destroys the first ones. At least not yet. When I wrote the description for the game I wrote something like use Shift for reset. Perhaps I should have used the word replay or something like that. That reset (or replay) is just a quick fix because I didn't have the time to make the game ready.
I would like to mention that in my opinion if the player gets confused by something when playing the game it's not the player's fault. It's a flaw in design. In this case the game is not ready yet. There are many things to add or fix. Right now I don't have the chance to make the game ready but perhaps in a few months time things are different, let's see.