Sourdough bread tends to come out flat

Hi all,

I'm trying to understand how to make nice open crumb sourdough bread but they tend to come out rather flat.

I've had success only once with a recipe using 65% hydration and immediately thought I could increase the hydration as I prefer to work with wetter doughs when it comes to handling it. 

I'll try and use the last bread I've made as an example:

255g white flour

70g whole flour

255g water

autolysed overnight

I've fed my sourdough the night before with a 1:5:5 ratio and added 60 grams of it to the autolysed dough in the morning. 

from this point I've added 6g of salt after 30 minutes and then proceeded to perform a coil fold every 45 minutes for a total of 5 coil folds. 

I assume my sourdough is rather slow (maybe because it new?) but the bulk fermentation took approximately 11 hours. I took a sample of the dough out of the main and placed it in a tiny jar to be sure about the volume increase. I considered the bulk fermentation done when it doubled in size. 

After this I shaped the dough by laying it on a flowered surface, folding it "letter style" and then rolling it up from the unfolded size. I placed the dough in a banneton and left it to proof. Since it was rather late I left the banneton to proof overnight in the fridge and in the morning I did a finger test and saw the dough would still spring up but slower than the night before. I baked it in the oven on a steel surface with water vapour (same method which worked fine with the 65% hydration loaf) but as soon as I placed the dough on the steel, and wanted to score it, it started to slowly spread. Not too much but enough for the shape to be completely "ruined". 

Is this a case of underproofed bread? Or could it be due to the loaf not having enough strength, maybe the folds need to be done differently? What is the effect of doing folds later in the bulk fermentation stage? Should they be done in the first hour and a half? 

I'm making another loaf tomorrow and will update with pictures