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As above- you really need to perform a Micro Adjust on the lens in the 7DII body.
Your depth of field at 600 mm is very narrow. Every body/ lens combination is different too. A lens can be at -2 on one body, and at +14 on another body. It is the combination of many, many manufacturing variations.
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I use Reikan FoCal or Focus Tune to perform micro adjust and test on all of my lenses. It is nice to have quantifiable data to compare results, side-by-side. First, you perform a Micro Adjust on the 7DII. Then test for best aperture, and note the relative sharpness at each F stop. Finally, you can do a 10 shot calculation of the "shot to shot variation." How ACCURATE and how PRECISE your auto focus system is.
When trying to figure these things out, you need to reduce the variables. Put on a 50 mm prime. Shoot on a tripod, with bright light and a high shutter speed. Use a timer to reduce vibration. If you can, shoot with flash to eliminate **any** camera motion.
Compare single point, center point, single shot auto focus, to the Live View auto focus, as someone suggested.
I have had 2 Canon 7DII which were both fantastic. The camera has the best auto focus system and AF controls/ ergonomics of any camera I have ever used. The odds are that your camera is fine.
Long lenses are always an adventure. I tried to compare a Tamron 150-600 to a Sigma 150-600 C on a friends Nikon D810 for him in Florida. We shot and shot and shot, and couldn't get consistent results on the Sigma. It turned out taht the dryer was running in my 2nd floor condo, about 20 feet and 2 rooms away from the target ... But probably on the same beam.
I think it will be worth your effort to do concerted testing, just for your own piece of minds, if nothing else. If you stick with that lens, compare it first at 150 mm. 600 mm is always a trick beats.
Then, go read the 52 page Canon manual on the 7DII auto focus system. Even after owning the 7DII for 1 year, I spent a couple of days studying that, and found some great customization settings to make the 7DII much easier to use for wildlife
http://downloads.canon.com/camera/brochures/EOS_7D_Mark_II_AF_guide_CUSA_9-2014.pdf
(By using 2 different buttons, with different AF modes and settings, for back button autofocus. Combined with the C1, C2, and C3 user modes. Basically, you can have 3 AF settings in each C1, etc. mode. The default setting with no button press. What happens when you push the Back Button AF button, what happens when you push the "*" button next to it. Or more- turn the camera, and each can have different settings in portrait mode. Etc.)
Good luck! "Don't Panic", to quote ... someone ....
Cheers!
Michael
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