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A generous dollop of blood fish and bone and/or pelleted chicken manure every spring to get growth going then a top up of special clematis food from spring until flowering. Some clematis can take a year or two to establish their roots before putting on a lot of top growth and flowers. I have several like that despite having deep, fertile, alkaline and mostly loamy soil. When planting any new ones, scatter some mycrorhizal fungae (as for roses) on their root ball before planting at least 4 to 6 inches deeper than they were in their pot.
I can't improve on Obelixx's excellent reply. I'll just pass on the advice I've been following. The RHS advises that each year, in late winter or early spring, one should apply a potassium-rich fertiliser (such as Vitax Q4 or rose fertiliser), according to the instructions on the container, then mulch immediately afterwards with organic matter, such as well-rotted manure, leafmould or garden compost.
This seems a bit of a joke, given the wet weather we've had, but they say that clematis need to be wateredregularly in dry weather during their first few seasons. Watering to soak the root zone requires at least the equivalent of four watering cans per square metre, they add. Is it possible that the soil in your new garden is losing moisture a lot faster than in your old garden?