If the MC was growing well the past few weeks, it means the light and Co2 are probably sufficient... therefore that just leaves nutrients to be the deficient factor. The newly added plants may be soaking up essential nutrients much faster than the MC too, so you'll need to dose additional fertilizers to support their continued growth.
Seachem Flourish is quite "light" in terms of various nutrient concentrations, so you may need to dose more of it to be effective in your setup (the bottle recommended dosages are just general starting values, you have to adjust the dosages based on plant growth and mass). Just increase the fertilizer dosage progressively and observe the plants, it takes a while to take effect (the changes wouldn't be immediate). The goal is to see the plants start showing more new healthy growth and less melting old growth.
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