Q: I'm really underutilized at work. Not only is the work too easy, I also don't have enough to do. I like my boss, but how can I get her to increase the challenge level in my job?
A: Be sure you're delivering well on your current tasks, and then don't be afraid to push.
The inner game
There can be a lot of reasons people get held back. Some might pertain to you, your skills and/or your work style. So before you start making more noise about getting more responsibility, take a look at your current performance.
This isn't an easy thing to do. You might find that you have blinders about areas where you need development or you may be a bit defensive. In order to progress, you'll need to set this aside. Try thinking about your performance as an outsider would. Put yourself in your boss' shoes or look from a co-worker's point of view or at the impression you'd give if you were a movie character.
Also think about feedback. If you don't receive regular feedback, that's an issue that you need to address with your boss. If you repeatedly receive feedback about the same issues, then you need to assess your response to it.
Now take a step back and consider other reasons you may not be getting enough work. Does your boss have time to think about your situation? Are you perceived as a threat in some way, perhaps by a weaker performer? Is there just not enough work? Focus on understanding this so that you can approach your boss with appropriate next steps.
Finally, be clear with yourself about what you'd like. What would you like to be doing that isn't currently on your plate? How much more could you take on, what additional experiences might you need to have, and what kind of convincing might you need to do to make this expansion credible?
The outer game
Once you've thought this through as realistically and thoroughly as you can, talk to your boss. Bring the situation to her attention in a neutral and solutions-focused way.