As with the others, i tend to agree that it's overwatering, but that doesn't mean you've watered it too much, just that it hasn't been able to get enough.
The soils from the stores are always too compact.
Try buying perlite or other airy additives and making the soil like a third sandy instead of muddy like the basic gardening soils are.
Then it will establish roots better and drink faster and be able to grow.
Lifting the pot method to gauge how much water the soil has is one of the best methods.