Gnome does not catch my program notifications

Hi.
Few days ago I moved to NixOS and I find it pretty exciting.
Once I got my whole configuration ready, I started wrapping up my custom programs to make them run on my system: this is the case of PaperBoy, a simple C program which checks your email and fire a notifications if you have something to be read.
To do it registers a G_APPLICATION and calls g_application_send_notification: it’s a very simple GTK application fully compliant to the documentation given by the Gnome team. And also, I used it for several months on Budgie Desktop: now, on NixOS configured with Gnome as DE, it just does not get any notification displayed.
I built it using this configuration: paperboy.nix.
And what’s more interesting is that the application itself is running without any issue, it’s just like if that specific notification call is not catched by the system:

~ ❯ paperboy -c ~/.config/paperboy
[paperboy] configuration parsed: 1 accounts
[imap] url: imap://mail.davidepucci.it:143/INBOX?UNSEEN
[imap] 12 bytes retrieved
[paperboy] posta@davidepucci.it: 1 unread

Does anybody know what this could be caused by? Thanks in advance.