My first Ficus Benjamina, leaves are dropping
I bought a ficus benjamina about three weeks ago. It was a little one, about 10 cm (4 inches) tall, found in a supermarket. On the walk home, it was kept in the cold (it's winter where I live) for a good 20 minutes. The place it stood for about a week was on a north side windowsill, and it didn't drop any leaves. After one week I decided to repot it into something larger, where it could grow untroubled. After uprooting the ficus I had the pleasant surprise to discover that I actually bought three plants, which I transplanted then in three separate pots. The soil is Compo Sana for green plants and palms. I added to each pot about 3 cm (1 inch) of clay balls before putting the potting mix in. I shook the plants quite a bit to get the old soil off the roots (which in hindsight was not a good idea, since I read that they're quite stressful plants) and also cut them a little shorter before putting them in the new soil. After that I let them sit two days, without watering the new soil. the first few days they seemed fine, after which two of the three plants started to shrivel up and drop their bottom leaves. The one that hold its ground had the worse spot, getting even less light (which I found strange).
I'd like to mention that under the window sill lies a radiator (so it's quite warm for the plants around 25 degrees celsius). They don't get much light, since they sit next to a north window.
At this point I really don't know what to do to make them come back to life and I don't really know if I watered them too much or too little. As I said, one of the plants is still going strong and even showing signs of growth.
I put up a picture of my three plants for clarification.