Loaf dense in the middle - shaping issue?

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Hi bread lovers! My sourdough loaves always come out denser in the middle and a little flatter than I’d like. Is this a shaping issue or proofing issue? I’ve only been able to achieve that perfectly filled out dome shape once, and I have no idea what made the difference that time! What am I doing wrong? No matter how gentle I try to shape, I get the same result. Here is a pic comparing an end slice with lovely open holes and a centre slice with squished holes. 

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I sometimes have that same issue and just pulled a 75% whole wheat loaf out with dense streaks in it. I thought immediately it needed more proofing but I will await the experts' answers before being hasty.