No oven spring

Toast

I use the Tartine recipe to the letter over and over and my loaves still come out flat. 

I have a homemade proofing box that keeps around 85°F while I do the 30 minute stretch and folds u til it gets 30% bigger. It usually takes longer than the book suggests to get that big though. 

Then after I take the turn the loaf out of the banneton into the Dutch oven, the loaf spreads.

 

How can I get a better oven spring? 

Thank you!