Why won’t Botox fix this line that I have when I’m NOT making a face?

That’s a great question! Botox is a neurotoxin, meaning that it works its magic by blocking the signals that nerves send to muscles. Those nerves are trying to communicate to the muscles to contract and move, but when they can’t transmit that message, the muscles stay relaxed. This muscle relaxation is what creates the smooth appearance of the treated areas after Botox injection. 
If there is a line that exists when the underlying muscle is already naturally relaxed, Botox unfortunately cannot relax that area any further. These sorts of “at rest” lines are often found around the mouth, and sometimes under the eyes. But don’t fear! Hyaluronic acid fillers can be a great option for treating this type of concern.