Thanks for your advice. The warm place was right next to the furnace, which stays pretty warm this time of year although probably less than 90 F.
My method worked beautifully using kefir and yogurt as starters. I figured that if the probiotics in the capsules were alive they would grow, even if a bit more slowly than at their optimal temperature.
But what really got me was the manufacturer's response -- like it's normal for probiotics not to grow. Does that mean they won't grow in my body, either? If so, what's the point?!