The "extra hit" of the reflex potion only works if your first attack strikes before the monster. If the monster hits you first, then you don't get the extra hit. This makes the reflex potion largely useless for most slowed characters; you will need to slow the target monster to get it to work correctly, which is a luxury most characters do not have (Earthmother and WEYTWUT are your only reliable methods)
This particular intricacy with the reflex potion can be a little unintuitive, and for the longest time I'd dismissed the potion as largely useless because I didn't understand its effect. They've improved the description text since then, but it's still one of the game's most fickle effects.