Fluorochrome (fluorescein) emission decay when linked to macromolecule
There should be no FRET for the free fluorescein unless you have specifically added an acceptor and in any case this would reduce the fluorescein emission.  When bound to the protein fluorphores are in a different environment (e.g. pH  change, hydrophobic pocket, tight binding, polar/non-polar nearby residues etc.)  and this can affect their fluorescence yield and lifetime up or down, ANS (anilino naphthalene sulphonate) is a good example.  The same type of thing is probably happening in your case, you know the motion is restricted as the polarisation is greater so there has to be some interaction. Fluorescein emits at a relatively long wavelength so aromatic protein residues cannot act via FRET as they absorb in the UV. You would need a chromophore (such as heam/chlorin/porphyrin) that absorbs in the green to red region to be a FRET acceptor from the fluorescein and so reduce its intensity.