I suspect I may have over-watered. It was very difficult to tell how wet the soil was deep inside the bags. On closer examination, I think they hold water really well and don't need to be watered that often. I also think I should have planted the seed potatoes sooner. Next time, I will buy seed potatoes from a Southern California source. I have a feeling they'll be available earlier around here than they are from Seed Savers in Iowa. (Does anyone know where I can buy seed potatoes in Los Angeles, preferably somewhere east of Vermont?)
So, this year was a complete potato-growing failure. On the bright side, when I dug into two of the bags I found this:
The bag was chock full of healthy, wiggly worms--and their highly-prized poop! While failing to grow a single potato, I managed to create a surprise worm farm. There were a few in the compost I added to the potato bags in June, and in the warm, wet, composty environment of the bag, those worms multiplied.
I took this rich mix of soil, worms and their poop and spread it on one of my garden beds. Hopefully the worms will be happy there, and continue to poop and procreate for years to come.