Flat flat sourdough (via Tartine recipe)

Hi All,

I'm a long time reader of The Fresh Loaf, but finally posting as I'm running into a problem I've not had before.

I've been baking variations on the Tartine country loaf for about two years now, all mostly successful (minus the occasional few I've let it over-proof out of laziness). Unfortunately during a recent busy period I neglected my sourdough starter and it collected a bit of not-so nice mold. So I developed a new rye starter about 11 days ago, fed daily with 50/50 wheat/white. 

All smelled good and looked good with the starter, as did the levain. However the bulk fermentation went much slower than usual. Normally I proof in the fridge overnight, but I was in a rush, so I proofed after bulk fermentation + 4 hours of resting in a banaton. However when I went to bake, it came out after the first 20 minute steam like a flat pancake with no oven spring.

Nevertheless, I decided to continue resting the remaining dough in the fridge another day to see if it maybe needed a bit more time, but once again I've got a pancake of a loaf (albeit good future crouton material).

Any thoughts what went wrong? Could my new sourdough starter be a bit off its game?