Again, like your last questions, without a diary and/or a heap of extra info, answering this question without knowing what you might be or might not be doing and expecting an accurate answer is almost impossible.
Incorrect feeding..........possibly
Incorrect pH...........possibly
Pathogens in the medium (if re-used) .............possibly
Not enough light..........possibly
Too much leaf removal.........possibly
Disease...........possibly
Poor watering habits...........possibly
A combination of two or more or even all of these things...............possibly