Possibly overwatering 'em. How often do you water them and what about nutrients ? Are you using any? What type and how often..? Because the leaves look otherwise healthy, no burn spots or yellowing, I'm guessing just by looking at the pics that it is overwatering. Nothing that can't be fixed. Learn to judge by lifting your pots, dont feed them until they feel light to you.
....Anybody else, free to chime in, I sure aint the last word !!