Answer: There are several possible reasons for this to happen. They range from overwatering (look for blackened roots or sour smelling soil) to underwatering (is the water running out between the side of the pot and the soil? is it rootbound?) to insect problems (check to see if the plants have any signs of webbing or very tiny insects, especially on the undersides of the leaves and around the stems) to even possibly a buildup of minerals over time due to fertilizers. If you suspect this last, leach the soil by drenching it and letting it drain several times. I hope this helps you troubleshoot the problem.
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