Comment 122 for bug 1574120

One other note, it appears that gsd-power might be blocking the suspend:

josh@josh-ThinkPad-T460 ~ systemd-inhibit
     Who: gdm (UID 147/gdm, PID 3040/gsd-power)
    What: sleep
     Why: GNOME needs to lock the screen
    Mode: delay

     Who: NetworkManager (UID 0/root, PID 1524/NetworkManager)
    What: sleep
     Why: NetworkManager needs to turn off networks
    Mode: delay

     Who: josh (UID 1000/josh, PID 20754/gsd-power)
    What: sleep
     Why: GNOME needs to lock the screen
    Mode: delay

     Who: gdm (UID 147/gdm, PID 3037/gsd-media-keys)
    What: sleep
     Why: GNOME handling keypresses
    Mode: delay

     Who: Telepathy (UID 1000/josh, PID 4930/mission-control)
    What: shutdown:sleep
     Why: Disconnecting IM accounts before suspend/shutdown...
    Mode: delay

     Who: josh (UID 1000/josh, PID 20754/gsd-power)
    What: handle-lid-switch
     Why: Multiple displays attached
    Mode: block

     Who: gdm (UID 147/gdm, PID 3040/gsd-power)
    What: handle-lid-switch
     Why: Multiple displays attached
    Mode: block

     Who: gdm (UID 147/gdm, PID 3037/gsd-media-keys)
    What: handle-power-key:handle-suspend-key:handle-hibernate-key
     Why: GNOME handling keypresses
    Mode: block

     Who: josh (UID 1000/josh, PID 20741/gsd-media-keys)
    What: handle-power-key:handle-suspend-key:handle-hibernate-key
     Why: GNOME handling keypresses
    Mode: block

     Who: UPower (UID 0/root, PID 2813/upowerd)
    What: sleep
     Why: Pause device polling
    Mode: delay

     Who: ModemManager (UID 0/root, PID 1466/ModemManager)
    What: sleep
     Why: ModemManager needs to reset devices
    Mode: delay

     Who: josh (UID 1000/josh, PID 20741/gsd-media-keys)
    What: sleep
     Why: GNOME handling keypresses
    Mode: delay

     Who: GNOME Shell (UID 1000/josh, PID 20651/gnome-shell)
    What: sleep
     Why: GNOME needs to lock the screen
    Mode: delay

13 inhibitors listed.

And this related bug https://github.com/systemd/systemd/issues/7137

In the gnome power settings, on 16.04 there was a toggle to have it perform suspend with the lid closed while docked or while an external monitor was connected. That option is now gone.