Answer: If you've been growing African Violets for 10 years, I'd consider you an expert. I suspect that your largest plant is simply growing old and declining, regardless of the excellent care you're giving her. You may want to take a final offspring from her and then remove her from your collection (I know it sounds harsh, but an ailing plant can often be more trouble than it's worth).
Just as a reminder: African Violets can be coaxed into blooming by giving them exactly the proper environment. Here are some secrets to successful growing: Give the plants average household temperatures, but avoid drafts or sudden changes in temperature. Place the plants in a bright, sunny window, but keep it from getting direct sunlight. Keep the soil moist, but wait until the soil surface begins to dry before watering. Use tepid, not cold water, and try not to get water on the leaves or crowns of the plants when you apply water. African violets like humidity in the air. To increase humidity, place the pot on a tray of pebbles in which you keep about a half-inch of water. As the water evaporates it will add humidity. Some expert gardeners are able to get African violets to bloom ten months of the year. This is because the plants are given perfect growing conditions. More often, though, African violets will bloom for a few weeks, then rest for several weeks, and then bloom again. Cut the flowers and stems off when the flowers fade.
Good luck with your plants.
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