In some instances compounds exhibit low fluorescence when concentrated. However, higher fluorescence intensity is observed when in dilute solution.
I such high concetration cases, the molecules will not be truely dissolved (not each of them completely solvated = entoured by solvent molecules), but still may form aggregates of molecules. While diluting, you will get full dissolution.
This is called concentration quenching, and poor solubility is only one of the mechanisms leading to the effect. Let's consider forming dimers and aggregation as particular examples of that mechanism. Other mechanisms include exciton annihilation, energy transfer, charge transfer, excimer formation...
One other possibility is that your compound is so concentrated that the light is absorbed before it has a chance of reaching the observation volume. Aside from that, there are the various incarnations of self-quenching, as mentioned in the previous answers.
While diluting any organic compound, it gets solvated due to breaking of bonds. This results releasing of electron in the form of energy or Fluorescence.
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