If you cast iron plant’s leaves are turning a crispy brown and drooping it’s most likely thirsty, but if the leaves are turning dark brown and soft it’s watered too often. While these plants are pretty drought tolerant, you want to water when the top 50-75% of soil is dry. Water thoroughly and discard any excess water. Be sure the soil is never soggy as it can lead to root rot issues.
Your cast iron plant does best in low to indirect light. Lower lighting conditions will cause growth to slow, but too much direct light will cause the foliage to turn pale brown. Prune off any browning leaves and place the plant a little further away from the window to prevent sun damage.