Hello,
for Cs-137 it is not because of the library. Actually for Cs-137 sources you get the gammas via the decay of Ba-137m and that should give you the well known 661 keV. Then you have lines at about 31,8 and 32.2 keV with already much lower intensity + a bunch of even less visible ones between 4.4 and 37 keV.
In principle you could also simulate in air but with this you already provide a potential scattering partner.
In general, if you want to check what is happening in your simulations then I suggest to get rid of anything that can impact your results, and then systematically bring back the respective parameters one by one.
Cheers
Chris