the proof is in the... proving?

I have been making sourdough bread again -- which I like because it is so easy -- but since coming back in January I have been having a terrible time with the second rise. The sponge bubbles nicely overnight, the dough rises a lot during the dsay, and then when I do the proving stage it just sits in the basket as a lump of cold, solid dough. It rises a bit in the oven, but the bread is still very solid. Delicious, but solid. I am assuming that the problem is some combination of (a) the first rise is left to go to long and (b) the kitchen is too cold and maybe (c) the rye flour in it is retarding the rise.

So today I used only white flour (the starter is mostly rye), didn't let it rise as long, and rigged my oven into a proving drawer by setting it to what should have been about 30C. And now there is a loaf baking in the oven which actually rose a bit during the proving stage, which will be ready in time for dinner tonight.