Every writer has an Achilles heel, some subject they are frankly terrible at addressing. I curse fiercely my utter inability to describe lips.
Their color, their shape, the machinations they get up to, every single thing about portraying lips in words befuddles me. As a result, my character sketches are always a bit murky about the mouth. (No picnic for a romance writer, let me tell you.)
Bottom line: Identify your heel and cover it wisely.