Gurus of sourdough, I need help! Third time gummy bread. 🥲 

So, a couple of months ago I decided to try sourdough. Because I like pain (well, not really, I just didn't realise buying starter was a thing) I made my starter from scratch. It is now six weeks of daily feedings old (wholemeal wheat), still a baby, but bubbly, active and smells nice. 

But. It does not fluff the dough up during bulk fermentation, like at all. I check on the dough every couple of hours and it baaaarely grows in volume. After 10 hours (at 23 C / 73 F) it grows by like 15%. And fair enough, when I bake it, it's a gummy mess.

My latest attempt was:   
350g bread flour (12% protein)  
230g warm water (65% hydration)   
80g starter   
7g salt

Mix for 10 minutes like my life depends on it. Wait. Stretch and fold. Wait. Stretch and fold x3 more times. Check on it every couple of hours. Give up after 10 hours. Pre-shape, bench rest, shape, cold proof overnight. Score, bake (220 C / 428 F, 30 minutes with the lid, 15 minutes without). Cry in disappointment 🥲 (But, to be honest, the taste wasn't bad - I still binned it, too gummy). 

Should I toss my starter? Try different flour? Bulk ferment even longer? Give up on sourdough? I've watched hours of tutorials and cannot figure out what I'm doing wrong. 

Also when I spiked my dough with some commercial yeast last week it turned out fine. So maybe it IS the starter? But it looks and behaves (in the jar, at least) so well! 

Thank you in advance ❤️

Old story, flat loaf