Total un-yoghurt

I had another go at yoghurt using the big earthenware pot now that it's summer -- total failure. This time it hasn't even begun to set. I was very careful with the temperature and with insulation, and I don't think it was either too hot or too cold. I defrosted my cube of 'starter' well in advance, and mixed it carefully with warm milk before adding it, so it should have been well diffused. The only possibility I can think of is that it had been in the freezer
for too long (the same batch worked the last time I tried) or that I should have put two cubes in instead of one, given the larger volume of liquid in the pot as versus the Thermos. But I would expect the culture simply to breed and spread, like yeast. (You are warned against using too much starter.)

Meanwhile I have a whole litre of sour milk to deal with...

[Edit: I've tried reheating it with an additional cube of starter as per my earlier attempts: Adventures in yogurt-making]