This is normal. The plants are re-using the water you gave them at the start. You should rarely have to add more moisture to the terrarium. It is basically self sufficient. Remember though, depending on which plants you choose will dictate where you place your terrarium.
Some plants love sunny windows and others love shade.
Always read your plant s care labels carefully and follow the grower s recommendations and your plants should flourish beautifully for you. Inc. PageWise makes no representations about the content published on this site. PageWise hereby disclaims all warranties and conditions with regard to this information, and user agrees that all such use is at its own risk. If you are under 18, please do NOT opt-in to receive email offers on this page.