If "flourish" means to thrive or prosper:
* Wither
* Decline
* Fade
* Fail
* Diminish
* Perish
If "flourish" means to be successful or popular:
* Fail
* Flop
* Tank
* Stumble
* Struggle
If "flourish" means to make elaborate or showy gestures:
* Hesitate
* Stammer
* Fumble
* Stumble
* Be restrained
If "flourish" means to wave something around dramatically:
* Conceal
* Hide
* Suppress
* Hold still
To choose the best antonym, consider the specific meaning of "flourish" in the sentence.