If you talk about the specular reflectance, a common reason is surface roughness... to give a more extensive answer, we would need more information about what kind of sample you measure in what spectral range by what method and instrument etc....

dear sir,

we have done multi layer thin film coating on optical glass substrate (BK7) and measured the reflectance spectra using UV-Vis spectrometer. the calculate and measured spectra were compared. we observed that there is decrease in amplitude with calculated one.

Ok, so how did you perform the calculation? The way to go would be to use Abeles formalism or Airy's formula while taking into account the thickness of the BK7 glass and that of the different layers and all the different dielectric functions / complex index of refraction functions.