Radiant energy absorption:

At higher concentrations of the analyte, as the radiation is absorbed through the analyte, the successive layers of the sample get progressively decreased intensity of the incident radiation. This reduces the signal of fluorescence. Therefore, this problem could be eliminated through dilution of solution or adopting standard addition method. You would recall in which we discussed about the standard addition method on UV visible spectrometry.

Self absorption

The other mechanism causing the deviation from the linear dependence on concentration at high concentrations of the analyte is self absorption. A few analyte molecules within the sample absorb the radiation emitted through other analyte molecules leading to the decrease in the fluorescence intensity. Within such condition also diluting the sample proves useful.