Need answers about my plant.

[Snagwood]

I have a plant that is 4 weeks old. Growing in DWS. This is the second time this has happened to me. The stem just above the Rockwood cube gets sick and does grow properly. Over night the stem doubled in size except for the bottom part. It will eventually just fall over. Any ideas what's wrong ?

 

[ceoradio]

Cover up no light to roots