Laptop doesn't suspend after closing lid

Basically the same question as asked in
https://askubuntu.com/questions/789846/

I.e., with my recently installed Lubuntu (18.04.2), the system does not suspend after closing lid of my laptop.

sudo systemctl suspend cam make my laptop suspend, so I believe it is Lubuntu’s configuration issue.

I tried fixing the problem myself by following
http://tipsonubuntu.com/2018/04/28/change-lid-close-action-ubuntu-18-04-lts/
i.e., uncomment the line #HandleLidSwitch=suspend from /etc/systemd/logind.conf:

$ grep HandleLidSwitch= /etc/systemd/logind.conf 
HandleLidSwitch=suspend

and I’ve restart the Systemd systemd-logind.service service as well.

But my laptop still doesn’t suspend after closing lid.
Please help. thx