Thanks for the details — sounds like you're doing a lot right. Since one lady is struggling while the others are fine in identical conditions, here are some possibilities and steps to consider:
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1. pH Too Low
5.6 pH is more in the hydroponic range. For soil, optimal pH is 6.2–6.8.
At 5.6, nutrient lockout can occur, especially calcium and magnesium.
Solution: Raise your water/feed pH to 6.3–6.5.
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2. Genetic Variation
Even clones or same seeds can show phenotype differences.
One plant may be less tolerant to pH, nutrient levels, or water retention.
Solution: Adjust care slightly for the weak one — maybe water less often or more gently.
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3. Root Health / Pot Drainage
Overwatering or compact roots can suffocate one plant more than others.
Check if the pot feels heavier (retains water) or lighter (dries too fast).
Solution: Ensure drainage holes are clear; maybe gently inspect roots if it worsens.
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4. Light & Air Circulation
One plant might be getting less airflow or uneven light.
Solution: Rotate pots regularly and ensure no plant is shaded or stuck in a humid corner.
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5. Pests or Disease (Check Underside of Leaves)
Early issues may affect only one plant at first.
Solution: Inspect closely for pests, spots, mold, or root issues.
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Action Plan:
1. Raise pH to 6.3–6.5 immediately.
2. Check for overwatering or soggy soil.
3. Observe for signs of pest or fungus.
4. Give the struggling plant a bit more dry time between feeds.
5. If you're seeing specific symptoms (yellowing, wilting, brown spots), send a pic or describe for more targeted advice.