Perplexed - my oven spring has gone flaccid.

Oh, no...not another oven spring post.

Yes, but I hope this might intrigue the more experienced members of this forum because the cause -- at least to me -- is not so obvious.

I have "beach ball" oven spring envy -- those loafs that look like they are just bursting out of their crust. I have been making sourdough for several years but it is rare that I get such a result. Not bad, just not great. 

My recipe can vary a bit but generally it is typically:

- strong bread flour with up to 15% "other" (spelt, whole wheat, rye

- 70-75% hydration with an hour of autolyse

- 20% very active yeast (has at doubled at least twice with two feedings). 2% salt

- four sets of stretches and folds or coil folds over first two hours. total bulk fermentation typically 4.5 hours but I generally go with the condition of the dough. Usually a 30-40% rise, jiggly, good distribution of gas bubbles, dome top, etc.

- pre-shape on counter, rest for 20 minutes, final shaping (sometimes really get surface taut, other times less so, but no appreciable difference in result)

- 10-12 hours cold ferment.

- have done hot start, or hot oven/cold DO at 475-500 for first half, 425-450 for second half uncovered. 

The result, as you can see, seems to have a good distribution of varying holes in the crumb, nice crust with defined ear, blisters, etc. But it just lacks that dramatic rise. 

Any of you who have arrived at this Nirvana, can you share what has been the biggest factor in your success or see something I'm missing?

Thanks in advance. If anyone is able to crack this I will gladly reserve s pick of the litter from the feral cat in our neighborhood!