Answer: Based on your description, it's difficult to say what the problem could be, however it is possible that they have reached the majority of their mature size and will not keep growing at as fast a rate as they did earlier. Perhaps you are overwatering the plants in the larger containers or that there is a soil difference between the containers. Plants in larger containers will not need as much watering as plants in smaller containers will. It is also possible that the initial richness of the potting soil has now been depleted, so that the plants are really relying on the fertilizer alone. (Purchased plants are often supplied with some fertilizer already in the soil, so that may now be used up, too.) You might try using a "blossom booster" fertilizer with a higher middle number and perhaps some compost tea to see if that helps. Finally, it is possible that the texture of the potting soil has changed so that it is compacted or has lost its ability to maintain a moist yet well drained root environment. You might investigate that and if this is the case, consider using a different type next time.
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