Dear Georgiana,
I managed to run your input in order to obtain the PHOTON spectrum passing from WINDOW to DETECT region without changing anything (by using the latest release of the FLUKA code). Your second USRBDX card will produce an empty output because the two regions do not share any common surface.
What version of FLUKA are you using? If you are using an older version, I think it requires the introduction of an empty LOW-PWXS card. Did you get any error in this direction?
Please be cautious, as scoring PHOTONs will tally all photons regardless of their generation mechanism.
If you want to be sure that you are scoring only Bremsstrahlung, please take a look at this post:
Please let me know if you have any further questions.
All the best,
Mihaela