Is too much water
I've bought two plants, same species (although I don't know which exactly). My question is why one plant is so yellow and brown and the other one looks fine. I can send you a picture of the sick plant. Thank you.
This very well may be either to much or to little water.
Make sure the soil is watered, but not soggy. Roots that sit in water will suffer from root rot.
Your potting soil should be well draining and your container have adequate drainage holes.
Here are a few links with good information on watering.