why do some molecules fluoresce and others don't?

It depends on having states available to absorb energy, and suitible decay modes for releasing photons. If the molecule has many vibrational and rotational states avalable, it may not fluoresce.

So, if the molecule has an option of vibrational relaxation, then it will do that instead.... simply because it is more favorable? Also, does conjugation have something to do wilth fluorescence?

It's going to depend on the strength of the coupling. It may be that there are no transitions in the visible, or that they are not strongly coupled. In those cases you wouldn't get fluorescence.

 

I'm not sure what you mean by conjugation.

By conjugation i mean structures with many pi bonds..... like aromatic rings.

By conjugation i mean structures with many pi bonds..... like aromatic rings.

 

I'm not sure what effect that would have. It may give you a band structure, so that the fluorescence would have a wider spectrum, if they were involved in the optical transitions. But that's just speculation.