Please take a look in the systemd Journal – “journalctl” …
For example, I notice that, I have some entries like these –
Jun 15 17:46:33 systemd[24469]: Failed to start app-flatpak-org.signal.Signal-838.scope.
Jun 15 17:46:33 systemd[24469]: app-flatpak-org.signal.Signal-838.scope: Unit entered failed state.
Jun 15 17:46:33 dbus-daemon[24489]: [session uid=1000 pid=24489] Activating via systemd: service name='org.freedesktop.portal.Flatpak' unit='flatpak-portal.service' requested by ':1.248' (uid=1000 pid=843 comm="/usr/bin/xdg-dbus-proxy --args=40 ")
Jun 15 17:46:33 systemd[24469]: Starting flatpak portal...
Jun 15 17:46:33 dbus-daemon[24489]: [session uid=1000 pid=24489] Successfully activated service 'org.freedesktop.portal.Flatpak'
Jun 15 17:46:33 systemd[24469]: Started flatpak portal.
Jun 15 17:46:33 systemd[24469]: Started app-flatpak-org.signal.Signal-865.scope.
As far as I recall, “Signal” started OK but, the functionality wasn’t quite that which the mobile telephone version gives, which is why I removed it but,
The above example may well supply some clues as to where you can begin searching.