Trying cleaning the shoulder buttons with a clean, dry toothbrush, but if that doesn't correct the problem, you might have to call Nintendo's 1-800-255-3700 (6 am ~ 7 pm Pacific, everyday) Customer Service number, or use their webform to set up a repair.
Buttons are sticky or are stuck down Having difficulty only pressing the "Power" button down? If so, click here. For all other buttons, try the steps below:
What you will need:
1 clean, dry toothbrush (no toothpaste on it)
How to clean the buttons:
Turn off the Nintendo DSi system. Remove the AC adapter, and any Game Paks and SD Cards.
Turn the system screen-side down.
Using a clean, dry toothbrush, gently and thoroughly scrub the area around the buttons, working the bristles around the sides of the buttons.
Test the system for proper response.
Repeat the process if necessary. If cleaning doesn't solve a sticky button problem, the unit will need to be repaired. We do not advise opening the system or using liquid cleaners at any time.
If the steps above don't solve the problem, your system will need to be repaired. Please click here for repair options.