Why example B3a do not reproduce X-ray of some nucleis such as Cs-137?

I am making a simulation of Edep in a scintillation detector of NaI(Tl). I am making that through editing exampleB3a. However, when I simulate Cs-137 or Eu-152, photopeaks of X-ray do not have enough counts.
Someone who can help me with this problem?