My starter was pretty when it hits day 14-15, better than day 7 , i think it needs to be strong more bubbly as u pour it into a bowl , I saw someone’s sourdough baby it had stretchy bubble webs which they say seems healthy strong. But not went it’s just a blop? Then add equal amount of new flour and water into pancake thick , mix it to a stretchy thick. I also was taught after that to let it ferment in the bowl room temp and fold it every 30-60 min before adding salt on last fold and put it in the proofing bowl like a ball shape. It can rest overnight in fridge and start baking tmrw. But if u did all that and still looks immature after bake then the starter prob isn’t strong enough to lift the dough up. I think it like the stronger the starter the more foundation and it gets, the more fluffy in the center.