Like @Murelli notes, staying too wide limits knee travel. If you're having a brutal time figuring out stance, I found doing front squats really helps. Why? Well you cant see your feet, so you have to get the "feel" for stance down, they help with hip and ankle mobility, and they put you as vertical as possible. All of these transfer well to cleaner back squats and consistent form, in my own experience.
References here:
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