Answer: It might be related to underwatering (if you have changed your watering routine or the plant has become well overgrown for the pot size for example) or dry air, however the most common causes would be fluoride in the water or a build up of fertilzer salts in the potting mix. The soil should be evenly moist but not wet, and it is best to avoid hot drafts during winter. If your home is centrally heated the indoor air can become very dry during winter so you may be seeing the effects of that. A humidifier, setting the plant on a tray of pebbles and water, or moving it to a cooler room would all help with that. If there is fluoride added to your water you should use rain water or distilled water to water your plant. If you have fertilized the plant over the years, there may have developed some residue from that. This can be leached out of the soil by watering heavily to the point of saturation and allowing the excess to drain and repeating several times in quick succession, then resume your normal watering schedule. I hope this helps.
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