Very likely because some interactions occur between the film and reactive groups in the organic compound. If this is the case, is much better the solution, but you must be sure that it is not particulate.

I would suggest to measure in solution as in thin film you will more likely experience the photobleaching and excitation wavelength dependent emission.   

both type of fluorescence measurements are important (solution and thin film) depending upon how your material is going to be used during its practical application. intermolecular interactions are different in solution and solid state. depending upon structure and functionality of molecules, they can show different spectral properties in thin films. fluorescence of molecule can be entirely quenched in film (concentration quenching) or can even increase (aggregation induced emission enhancement AIEE) along with red or blue shift in spectra.