Sudden Death!

This is the second time that this has happened in the last 30 days. One day everything is all good then the next day one of my plants looks like there might be some watering issues. (over-under). i dig a small hole about half way from the stalk to the edge of the pot and everything feels fine. take a hand full and compress it to see how much it clumps. everything looks great. however b/c i am not able to see the root ball i decide to see if letting it dry out more might help. 24 hours later i check the plant again hopping that it is back to its normal healthy self. it was not. curiosity got the best of me so i pulled it up to see if there was maybe some root rot. other then not seeing as much roots as i would have hoped to see everything looked normal. all the roots were white. the only thing that i noticed that was "wrong" was that at dirt level the stalk looked healthy however about an inch down the stalk it was much smaller in diameter then it should have been. i broke open the stalk to see if i show any signs of pest but nothing. i now have another plant doing the same thing. i am so lost as to what could have caused this, how to save my plant, or how to keep this from happening to anymore of my girls.