More info needed.
Indoor potted plant? By window? Hot sun?
Ever fertilised? What is the growing medium?
Where are you geographically?

Hi,

The plants are in pots and I keep them indoors during winter and outside during the summer. I live in Bulgaria, so we have hot summers here. There is plenty of light on the place I keep them, but not too much sun, only in the mornings. At noon and in the afternoon they are in shadow. I use nitrogen fertilizer every week from about couple of months, because the leaves started go yellow. I have used soil available here mixed with some sand and put drainage.
There is one plant that is doing all right for now and I wish to save it.