The actual buttons are built onto the board of the Nintendo DS Lite. Sounds like the bumpers that push the buttons may need to be realigned or replaced.
Also, liquid damage could be a result of the ify situation your in. Take off the battery cover and take out the battery. See if the water indicator is solid pink. If so, the system has been exposed to moisture and this could be your main cause. If not, then it's what I listed above.
If you don't feel like messing with it, contact us and we'll be glad to help.