If the lens is pointed at something at infinity, and is currently set close to MFD, or viceversa, the AF system is simply unable to find any pase difference. This is especially true at longer FL's such s 600mm, and affects all lenses of these focal lengths. There are some AF settings in the camera body that affect what will happen in that situation. Either the AF system will drive to the other end of the throw, and try to AF again, or it will do nothing. This is set in one of the Cfn menus.
Personally with my 50D I have AF Drive when focus impossible to OFF. If I am shooting aircraft in flight and I manage to drop the AF point off the subject I do not want it going current focus-MFA-Infinity-original focus distance. With my old Sigma 28-300 at 300mm this process could take 2-3 seconds, although the 160-600 with it's HSM motor is a lot faster moving, the focus throw is still quite long. It is also useful if the sibject passes behind an obstruction while in AI Servo mode. More modern bodies have variable refocusing delays, so having the AF drive setting at off is less important.
So with the AF drive set to off if the lens is very out of focus it just sits there doing nothing. If you don't know about the AF Drive Cfn setting it can look like a lens fault. Even though I deliberatly set mine to off I still forget at times and think bad thoughts when the lens attempts to AF.