How can I make my computer suspend properly?

I have installed SolusOS (3) on my laptop. This laptop has a second monitor plugged in via HDMI. When I close the lid of my laptop the computer does not go into Suspend, the computer remains on as if I'm using just the external monitor. I don't see any settings which I can use to tell the computer to Suspend when the laptop lid is closed. If I unplug the external monitor the computer will Suspend, so it is the dual-head monitor set up that is causing this. It is quite normal to want to Suspend when the laptop lid is closed even with a dual-head monitor setup. But I can't find the setting.