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it might be a mix of no air circulation in there causing no oxygen flow, and you watering it too much. The over watering is what is mainly doing it though. To be more specific with an answer though, please post all the specifics about your plant.

Age
lights
soil
size of pot (plant container)

pics? because i had this same problem and if it looks the same i can help ya fix it

Leaves curling can mean several things...it can be a bit difficult pinpointing the exact cause

1. Over-fertilizing - the most common cause of leaf cupping (curling under) is the overzealous use of too much plant food. The leaves usually feel hard and crispy

2. High Heat - the plant is losing water via it’s leaves faster than what can be replaced by the root system. The leaf responds by leaf cupping or rolling up or down in order to conserve moisture.

3. Overwatering - Overwatering weakens the root system by depriving the roots of proper oxygen. It can cause all kinds of problems like leaf curling, stunted growth, and root rot

4. Underwatering - Underwatered plants will without a doubt wilt and the leaves will appear to be curled


If you could get some pictures that would help

pictures.....