There could be several reasons. Watering, to much or to little.
Don't water on a schedule, check the soil. Peace Lily will do best with slightly dry soil. They will hang their leaves when they are thirsty.
Watering with conditioned water can also cause the salts to build up in the soil. Try using distilled water or sitting the water out on the counter for 24 hours.
Here is a link to refresh you on the care requirements.