Why doesn't my 24-70mm focus at 24mm?

I found that my lens does something odd at 24mm. Its a canon 24-70mm 2.8L that won't autofoucs at 24mm. If I turn it slightly to the right, it works fine. But all the way out at 24mm, it just struggles to focus and will not lock onto anything. The only exception is when the target is something very close to the camera (like within 2-3 feet) in which case it will focus. But anything else that is further away won't work.

Thank you,

Sarah