DIAGNOSIS: Dried smoothie or cleaning liquid has seeped under the silicone membrane of the touch control, gunking up the contact. Yeah, this one's common.
FIX: Unplug it. Dampen a cloth with a mild cleaner (like dish soap and water) and thoroughly wipe the entire top control surface, working the cloth around the button's edge. Press the button repeatedly while wiping to work some moisture under the rim. Dry completely with a towel. Let it air dry for an hour before plugging back in.
IF THAT DIDN'T WORK: The membrane switch underneath might have failed. There's no official user fix for this. If it's under warranty, contact NutriBullet. If not, you're looking at using the pulse function (holding the button) as a workaround until it fully dies.
HEADS UP: Avoid spraying cleaner directly onto the base. Always pour cleaning liquid onto your cloth, not the appliance. A slow, unresponsive button is often the first sign of eventual total control failure.