>>280760834
Some of it is re-emitted as UV, it just happens with much lower probability. Excitation to higher vibrational levels is usually more common than a 0-0 transition ( the transition probabilities are determined by how the wavefunctions overlap at different inter-nuclear separation, it's called the Franck-Condon principle) and non-radiative vibrational relaxation is a much faster process than the electronic relaxation that produces the luminescence. There are also other things like how the vibrational levels are closer together in electronically excited states because of weaker bonds, de-excitation into levels other than the ground state, internal conversion and ISC etc.