Advice please

hi guys

i have a starter that I was given, feed it twice a day 10% starter 100g bakers flour and 55g water

starter is healthy and going well.

i try to make free shaped loaf at 70% hydration but flattens out in the proofing stage.

oven spring is also delayed so not getting any nice tears/ ears on the loaf. Preferment is 10-20% starter, 100 g flour and 100g water left for 1-2 hours on bench.

i use 20% preferment of my final dough flour weight, autolyse for half hour hold back 50ml water for double hydration when adding salt. I use the stretch and fold method over 4 hours then long ferment before preshaping. Bench rest for another 20-30 mins before final shaping. Tried bench proofing and in a commercial proving oven. Loaf just seems to flatten out.

any advice would be great.