Half of my potted citrus are fine and healthy with plenty of new growth. They are watered with rainwater when my water-meter indicates it is necessary (currently twice a week) and fed weekly (using Millet's sugested dosage). In the winter/spring they are kept in my heated conservatory and currently they are all outside.

Unfortunately my other 5 plants are less healthy. They have absolutely no new growth and the leaves look lacklustre. My grapefruit lost all its leaves in the spring (over about 2 months) and nothing has come back though the branches are still green while my calomondin has a large number of dull slightly off-green leaves (a number that have curled). These plants need watering every 10-14 days despite the weather in the UK being hot, dry and sunny. The roots do not appear that healthy. What can I do to stimulate leaf growth in these 'ill' plants?

Daniel