Why are you using such a low ISO? You can boost it much, much higher on the 6D without sacrificing image quality. Boost your ISO and get your shutter speed up. Even though you got great shots at 1/20-1/40 (which really impresses me), that's a huge gamble. Honestly, I don't know how you and your models pulled it off. I have no explanation.
I won't hesitate for a moment to use ISO 6400 on my 6D. I do have to apply some NR in post, but not enough to lose image detail.
I am used to shooting at 100 since I use off camera lighting. I'm really perplexed as to how I got great shots with a slower shutter then yesterday didn't fare as well. The only thing I can think of is the lighting may have been a little more dim yesterday as it was closer to sunset.. Is this possible?
The only thing is I am also limited to 1/180 flash sync speed by the 6D. Is this going to be a problem with banding?
Always sacrifice ISO for shutter speed. You should first make sure your shutter speed is enough to stop motion, noise is easier to clean up (especially on newer cameras) than motion blur.
I always try to go with 1 / (twice the focal length) at a minimum, and back off of that if I have to. But it is my starting point. At 200mm and 1/60th with no IS, you are asking for motion issues.
You could have gone to ISO 400, 1/240th very easily and your capture would have most likely been crystal clear.
ISO 400 1/640th f3.5 example... I could have gone down to ISO 200, where I did most of this shoot, but ISO 400 does nothing to the final results. I would rather have too fast a shutter at a higher ISO than too slow a shutter at ISO 100, especially with the 5D3 or 6D.
ISO 800, where we were mixing it up under trees, etc. and I never brought it down, so shutter speed went way up. No issues though with noise.]
Yeah this makes sense but I'm using off camera lighting so my ISO shouldn't need to be too high. Some of the shots are at 70mm 1/60 and still missed focus.