My stiff dough levain is no longer rising
I am very new to sourdough, having created my first (rye) sourdough in January, and I could really use your expert advice. I've been using my stiff dough levain since I created it (from my rye sourdough) 3 months ago, and, while it always took a full 12 hours to double, it always did double in the end. After reading posts on this forum in which people have gotten their levain to quadruple in 4 hours, I decided to keep my levain out of the fridge and feed it regularly until it became more active. Perhaps it was a mistake, but I was only feeding it once a day (our house is quite cool - 55 F at night and 60 - 65 F during the day, and the levain has been slow to rise). In the last 2 days, the levain has been changing - forming bubbles, developing a sour taste, and changing consistancy - after every feeding, but not rising at all. Because the house is cool, I have been boiling a cup of water in the microwave and then placing the levain in it - as a steam box, to warm it and accelerate growth. I used to find this very helpful, but lately it has not helped.
Any ideas? And can anyone recommend a book with the most sourdough detail, to help me troubleshoot when things like this go wrong?