Answer: There are several possible explanations for why this could happen. It might be as simple as the plants were a smaller variety and you should have used more of them, or maybe the plants might have been root bound or otherwise stressed before you planted them, or it might be because of cultural conditions. Probably the biggest problem is keeping the potting soil moist enough which means watering often and in some cases using a water holding polymer in the soil; lining the inside of the moss with plastic with a few holes punched in it at the bottom for drainage can also help since the moss "breathes" and water can evaporate through it. If you don't use plastic, make sure the moss is fully damp each time you water or it will wick water away from the soil. Smaller pots dry out faster than larger ones, so sometimes using a larger pot size can help. Wind and direct sun also dry them out. Frequent watering can leach nutrients out of the soil quickly so you also need to fertilize regularly with a water soluble fertilizer according to the label instructions. In my experience using a bit of compost tea or a seaweed-based fertilizer from time to time can also help.
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