Charcoal keeps the substrate fresh and the water from going mouldy. Add/scatter a few bits of natural charcoal to the substrate... (make sure they're not impregnated with a fuel-starter chemical...) Use fresh soil. Make sure ventilation is adequate.

The charcoal only works for a short time. Terrs like yours generally don't last for a long time. (Lack of air circulation is often on of the biggest issues.)

You possibly may have overwatered. That is a very common dilemma with that sort of terr.

A friend of mine accomplished - after a few false starts - the 'perfect' terrarium. It was, in essence, self sustaining, creating its own moisture and climactic environment. She had ferns in there and it was absolutely wonderful for around 3 years. Then she went away for a week with her mother, on holiday, while her hubby decided to surprise her and decorate the living room. He made the fatal mistake of moving the terrarium, thereby effectively altering its hitherto perfect conditions (which I'm afraid, I am unable to describe in detail...) and ruining it.