they need plenty of water and being in a pot that isn't always possible. the warm weather will make it lose lots of water.
these really are better in the ground though.

When you say compost, do you mean potting compost, or compost from your compost heap/bin? Are the pots much, much bigger than the rootballs were? And have you kept them well watered?

Big pots and potting compost. I'm going to try mixing some more soil with it and see if that helps. Many thanks for both of the replies and fingers crossed.

If the pots are more than three times the size of the rootballs, you need to use smaller pots till they get bigger - too much empty compost surrounding a plant isn't good because it becomes sour and can affect the health of the plant, or it seems damp enough, but actually isn't around the rootball. It won't be the compost causing trouble, if it was potting compost, and you don't need to mix ordinary garden soil in, that can cause problems because it isn't sterile and may contain pathogens. This answer assumes the pots have drainage holes which aren't blocked, and that you've been watering sufficiently for the water to run freely out of the bottom, and watering often enough.