Why does the excitation of molecules not take place during the stimulted emission process?
Excitation may not take place in a laser (or the like) for a
number of reasons. The lasing medium may be defective or
contaminated and unable to lase. The stimulation may be
insufficient or incorrect. There are other technical reasons that
prevent stimulation emission from taking place, but they generally
have to do with the lasing medium or the action of the stimulator.
Our friends at Wikipedia have a some relevant articles up, and
links are provided. Knowledge there is free.