I've never grown these, mostly because I'm on heavy clay too, but have had a look at the cultural info in my Encylopaedia - needs full sun and free draining soil, which probably also means they don't need much feeding. It may have been that the John Innes No. 3 was too rich for them. Another book I have suggests that full sun during the whole of the afternoon and evening is critical, as is protection from wind, which they don't like at all. They are also extremely thirsty, and need regular watering.
I don't know where you are in the UK, but the 'out of the wind' thing is very difficult now here in London - it's much windier than it used to be and much more frequently, particularly during summer I notice. As for the size of your pots, I'm assuming you mean 15 inches deep, which I'd have thought would be plenty. If you replenish the compost mix in them next year for these plants, try using John Innes No. 1 instead. Sorry I can't be more help.