please help: urgent; why don't all molecules fluoresce?

Because electrons in all atoms are not bound loosely, and can't be separated from it unless a large amount of energy is give to them.
one more thing, it is not necessary that an electron will transition to the ground state by releasing a photon of detectable wavelength.

if u have understood my explanation to your first question then u might have guessed that we need an energy source which can provide continuous energy to the atom so that the electron first transitions to an excited state by absorbing a photon and then goes back to the ground state by emitting a photon.