My experience of 'soft shots' is with a Canon 100-400 at the long end. It will only perform well when there is plenty of light so that there is good edge contrast to sharpen on. The max opening of this lens is 5.6 at 400mm and I get the best results with F6.3 to 7.1. Basically the subject has to be sunlit for it to work well.
I have never used the Sigma but I imagine it may be similar. The only way to get consistent sharp wildlife is with a prime lens. None of the long zooms seem to be up to it.
Pete
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