It's not that your game is too hard or too easy. You can dive in a little deeper. It's that your difficulty curve is not tuned properly. IMO you can tune it statically (follow most of the advice here, make it harder). Or, you can use some very basic dynamic difficulty shifting (If they complete the first level in >wpm, ramp up the difficulty, or adjust the difficulty curve as needed).
See this great video from Extra Credits on how Bejeweled 2's levelling system works. It's not the same problem, but it's very related. "A designer should always be thinking about how the player experiences the game."
See this great video from Extra Credits on how Bejeweled 2's levelling system works. It's not the same problem, but it's very related. "A designer should always be thinking about how the player experiences the game."
http://www.penny-arcade.com/patv/episode/puzzle-games