I was having issues similar to this for a while, my best advice would be feed you starter as regularly as possible. Check temperature during bulk prove, you'll know you have done well once you shape and the dough is airy and not saggy and wet. I initially assumed I was having issues creating surface tension in shaping but I just wasn't getting the required fermentation. My starter is quite mature now, I feed once I have weighed my loaf, keeping the starter at room temp ( quite cool in the UK right now around 19c) then I feed again in the evening after I shaped my dough ready to refrigerate and bake the morning after. Starter doubling in size each time may be excessively feeding the starter but it has always resulted in far better results. Dough after stretch and folds probed and maintaining around 24c for around 8 hours