take a bright flashlight, and look if you can see something under the keys when pressing them. use some tweezers or a toothpick to remove any dust/hair bread or chips crumbs or whatever. if you don't see anything under the keys, try blowing air into/under the key (in the us i believe "canned air" is quite common) if that doesn't help either, try to remove the key by levering it out with eg a knife, a scissor or something similar. CAREFULL if there is too much resistance maybe the keyboard is not constructed for that.
if all fails, and you are sure that you don't get any warranty form amazon - throw the keyboard into warm water, rinse, shake, repeat. get it dry as good as possible with some towel, dry it over a radiator (not too hot or the plastic will melt) or just somewhere else for at least 24 hours, check if that helped. if the keyboard refuses to work afterwards, you can try some spray that enhances that electrical conductivity. as i said - only as a last resort, this might make your keyboard completely disfunctional!