I've had my T6i for almost a year, but recently have been having issues with the auto focus. I can take a shot, then without changing what I'm looking at, go to take a 2nd picture. Instead of a fine adjustment it will go the go the full range of the autofocus to find the subject again. Often just ocelating back and forth for several seconds before just stoping, without being able to take a shot. I can get it to work again by looking at something else though. I've tried 3 lenses all Canon with similar results. I've tried some things I've read on here, like factory reset, cleaning the lens and camera contacts, using differnet shooting modes. I've been out in good lighting while trying to sort out the problem with no luck.
Go into the menus and try resetting the camera back to factory defaults.
What lenses are you using? What are you trying to photograph?
Which shooting mode are you using on the mode dial? What exposure settings is the camera using?
Which focusing mode are you using, One Shot or AI Servi? Which AF points are active?
I have experienced the same "hunting" on my T5. Usually it is caused by something in the near field or low contrast. The problem also shows in low light. Another factor is most cameras have a difficult time focusing on distant objects, such as picking up a center fielder against a chain link fence.
Many less expensive lenses don't have a large enough aperture when they are stopped down for the lens to adequately focus or give a exposure value. This could be noticeable when keeping your finger half way on the button. The initial shot is calculated with an open aperture. That aperture stays fixed if your taking another shot but the camera thinks the focus / exposure has changed.
If any of these situations might fit, a suggestion would be to take your finger completely off the shutter button in between shots. That will allow the lens aperture to fully open. If there isn't enough light, then try using the on-camera flash.
If you had lots of light then I hope you find the problem. My wife uses a T6i and thinks the world of the camera. My 80D hasn't had that issue.