probably crumbs or dust etc under the keys--try using a small paintbrush or can of compressed air to get rid of little bits of stuff that get under the keys--if that doesnt work you can gently remove the keys to clean under them--this isnt without risk as if you pull them off too hard you may pull the little scissor mechanism off as well and they are a pain to put back on--if you gently remove the key only they just push back down and click back on