Could be deadtime saturation. Reduce dose rate by diminuishing probe concentration or reduce background signal.

Regardless of the fluorimeter type there can be at least 4 reasons for a saturated fluorescence: gain is too high, concentration of the fluorophores is too high (if the gain is already at its minimum value), excitation signal is too close to the emission signal causing detection of the diffused light or the cuve/column is dirty with a lot of fluorophore contaminating your measurements (if you see a saturated signal even at a fluorophore concentration of 0).

If your gain and concentration are low, excitation is far away, apparatus is clean and the problem is still there, it's probably electronic related but I can't help you there.

 Agree with Julien, He has very good answer.