OVER THE WEATHER
Credit: Joseph Appel

UNDER THE WEATHER is an idiomatic expression meaning you're not feeling well. But OVER THE WEATHER does not mean you're feeling fine. Can you think of any other idioms that don't work when you use the opposite of one of the words?

(Possible) Answers:

DOWN THE DRAIN
IN THE PINK
OFF-THE-WALL
OFF YOUR ROCKER 
ON CLOUD NINE
ON THE BALL
ON THE FENCE
ON THE ROCKS
OPEN A CAN OF WORMS
OVER A BARREL
OVER YOUR HEAD
OVER THE RAINBOW
PLAY THE FIELD
PLAY WITH FIRE 
PULL STRINGS
PUSH THE ENVELOPE
SALT OF THE EARTH
SITTING DUCK
SITTING PRETTY
STRAIGHT ARROW
SWEET TOOTH
TAKE THE CAKE
TOP-DRAWER
TRUE BLUE
UNDER THE RADAR
UNDER THE TABLE
WALK ON AIR
WET BLANKET 

NOT BAT AN EYE