Too much water? Too little water? Too much sun? Too little sun?
I've had the little ones in the square pot for about 18m, and the bigger one in the round pot for about 5-6m. I give them a little water once every week or two. The windowsill is south-facing. When the sun is particularly hot I take them off the windowsill and move them into the shade. Last year I kept the little ones outside for the summer, also moving them into the shade when it was particularly hot. All that happened was two of the original four died.
Please help 