You may be experiencing the inner filtering effect. In a fluorimeter at high concentrations most the light gets absorbed by the sample before it reaches the middle. i.e. gets absorbed outside the detection volume. Therefore the amount of excitation light reaching the sample within the detection volume is decreased and the measured fluorescence intensity subsequently decreases. Primary inner filtering effects will distort the excitation spectra and secondary inner filtering effects will distort the emission spectra. You will see the shifts in more detail if you carry out an excitation-emission matrix. For a reference see here:

https://www.researchgate.net/publication/257599532_How_to_Collect_National_Institute_of_Standards_and_Technology_(NIST)_Traceable_Fluorescence_Excitation_and_Emission_Spectra

Its also possible that you have exceeded the linear range of the detector. FITC is quite a strong fluorophore check the specified linear range of your fluorimeter e.g. a HORIBA FluoroLog/FluoroMax has 2 million counts and then the response becomes non-linear and may begin to fall resulting in multiple peaks. See 4:15 Measuring Emission and Excitation Spectra using the FluoroMax 2 with Correction Factors and I explain the non-linear response of a detector in more detail:

http://dellwindowsreinstallationguide.com/instrumentation-strathclyde/steady-state-fluorescence-emission/

I have a video on measuring EEMs and I plan on doing one about inner filtering effects at some point.

Check with the manufacturer of your fluorimeter for a front face attachment if you aren't already using one, this may help you work at high concentration.

Its possible to correct the inner filtering effects by comparing with the absorbance spectra which the HORIBA AquaLog does.

Article How to Collect National Institute of Standards and Technolog...

Another possibility is that the fluorophore concentrations are too high and you are getting intra-molecular self quenching; see fig 6, 7 and 8 of:

https://www.lifetechnologies.com/uk/en/home/references/molecular-probes-the-handbook/introduction-to-fluorescence-techniques.html

dropping antibody concentrations should bring you back to the proportional relationship if this was the case.

Hi, thanks for the replies. The instrument used is Horiba Fluoromax 4. Will have a look at the solutions provided and sort this out. 

I think Philip and Ngee are referring to the same effect - the inner filter effect.

FITC is known to be quenched in the presence of antibodies. http://www.ebioscience.com/media/pdf/tds/12/12-7691.pdf

So, it can be a combination of the quenching as well as the inner filter effect. One way to test may be to use a better dye (such as Alexa 488) with the same antibody.