the switching system used is very sensitive to outside influence. if a drink was indeed spilled, it will throw off the sensing circuit. (the difference between the sensor open and blocked is only a few tenths of a volt difference at only a few microamps.) also, painting the button "flaps" so they block more light when pressed helps too. they often bleed light through and around the flaps and the phototransistor picks it up still.
if you use a digital camera, you can view if the infrared LED's are indeed on, if not, check upstream for a small resistor and check if you have voltage there. you may have to take a soldering iron and reflow all the solder as sometimes surface mounted stuff get's cracked/bad solder joints.