A simple enough question, do you keep a lens on your camera/s all the time or do you remove it/them when not in use? If you keep the lens/es on all the time you might like to consider this; the contacts that transfer data between camera and lens are designed as "wiping contacts". The point about wiping contacts is that they keep themselves clean by wiping across each other whenever a lens is mounted or removed. If you don't remove your lens/es the contacts never get cleaned. I recently had a problem with my 70-200 and TC17 where the camera ceased communicating with the lens. Removing and refitting the lens has solved the problem. I have got lazy and don't put the lens away after use, when I did that I didn't have problems. So, a suggestion, before looking at a repair to your camera or lens, just take it of and refit it a couple of times. Probably worth running a cloth over the contacts as well.