R5 Mark II is not properly focusing on birds eye

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Recently as I’ve been attempting to photograph birds, I have been noticing that my R5 Mark II struggles to correctly focus on the eye of the bird, and sometimes the bird itself. To my knowledge, I have all of the correct settings in use (based upon multiple YouTube videos and google searches), yet still struggle to get good focus. I am using a Sigma 150-600mm Contemporary lens with the EF to RF lens converter with focus ring, so range is certainly not the issue. Even though the bird itself tends to be in focus, the eye is just not consistently in focus and as sharp as other parts of the bird. 
I use eye tracking autofocus and whole area autofocus typically with servo. Usually, in the viewfinder, the eye of the bird is in a blue box, but I still feel like the images are not in focus that well. Any suggestions? Thanks! 

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Another vote for the 200-800.  Some amazing eagle shots there Royjoyinc.  Recently took just the 200-800 on a vacation to Florida to get better with it and the camera.  It was fun and a good education.  One of my big lessons learned was to get more comfortable with higher ISOs to get to faster shutter speeds.  Caught some good bird photos, but haven't downsized them under the 9MB limit yet.  The lens does a great job when I do my part!

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Funny enough, I did end up buying the 200-800 a bunch of months ago! It has been a really great step up!