Weak Marijuana stems

A question from a fellow grower:

the stems appear weak -what might the problem be ?

Day 2 after germination -1/2 inch stem w/2 leaves -
Day 9- 1 stem was at a 45 degree angle with a bend about 1/2 inch form the 4 leaves -
2 stem -was completely limp and laying flat-

It’s called damping off: