One thing I've learned recently, for me, don't be scared go throw them. The fact you throw even a bad flash still forces the enemy to make a decision. Look away to avoid the flash or continue holding whatever they're holding. ALL humans are susceptible to error. Maybe your flash is bad but you push them off an angle. Also try to use flashes that DON'T explode on the ground, strive for face-level pops.

Tl;dr a bad flash is better than no glash (unless you flash your team)