are you letting the soil dry out inbetween waterings....root rot?? You dont want the roots to always be wet.

I usually let my plants leaves start to droop just alittle and the dirt will pull away from the potter too...then I water heavy...this provides a healthy root system in dirt

You could always get a moisture meter this will help alot under $10 3in1 Hydroponic Plant Soil pH Moisture Light Meter - eBay (item 350133892471 end time Dec-03-08 19:42:31 PST)