the smaller muscles in your shoulder likely aren't up to the task of keeping the shoulder stable under that much weight. I've had the same problem occasionally when I've tried to aggressively increase my bench. i would eventually be more limited by the shoulder pain than the strength in my chest and arms.

So I'd suggest reducing weight and volume to something you can do with no shoulder pain and adding some lightweight, high rep rotator cuff exercises to your routine (I do mine on chest day). Then take your time adding weight and volume. It took Fletcher and them Years to get to the point they can withstand the kind of volume they endure.

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