Something inadequate doesn't cut it. If you lack just a few centimeters to reach the low mark on the wooden sign, your height is inadequate for riding the big roller coaster. Wear heels next time.
The Latin root of adequate means "equal to," so inadequate refers to things that aren't equal to what is expected or needed. Something can be naturally inadequate, as in "inadequate rainfall to stop the fire," or it can result from lack of effort, as when someone avoids a question: "Your response was totally inadequate. Tell me what really happened."