How to make eyebrow hair grow back?

Hi, I over plucked my eyebrows years ago (5+) and have since been filling in the ugly little gap in my eyebrow with pencil. I'm sick of this and really want my eyebrow to grow back! It seems the eyebrow hair has no trouble growing where I don't want it, but in that little spot it's never shown any sign of returning. Is there something I can do to encourage the hair to grow back there? Thanks :)