Clematis not developing

I grew clematis in a slightly acid heavy soil successfully for years. Since moving to a light, neutral soil four years ago I have planted several clematis. The established one from my old garden still flowers but has very little new growth. Others have barely survived the winter and just manage to replace the little growth they had when first bought before struggling during the next winter. Two plants, usually easy to grow President and Montana bought as fairly large plants died. I plant them with extra compost and shade with a small plant. They are in a variety of positons , mainly South East facing. What should I feed the plants with to encourage strongergrowth?

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?

Thank you Gardening Granma and Obleixx most helpful.