Use the adjective ineffective to describe something that doesn't work quite the way it's supposed to. You might complain about your ineffective guard dog, who wags his tail and licks everyone who comes to your house.
When you're ineffective, you're unable to accomplish something you're trying to do. You might realize that you're an ineffective babysitter when you can't get the kids to go to bed. Ineffective comes from the Latin prefix in, "not, or opposite of," added to effective, whose Latin root is effectivus, "productive." When something's unproductive, inefficient, or incapable, it's ineffective.