Do you keep a humidity dome on them?
How many nodes do they attain before death?
Are the yellow leaves just plain ol yellow, or do they also have dead tips or purple veins?
Is your water out of the tap or from a bottle? If it is bottled, what is the brand and type?
What is the air temperature right at the soil surface?
Tell me about your pots, their drainage, and the watering schedule. How do you decide when to water?