Systemd will delay suspends after booting in order to check for docks and external monitors. The laptop should eventually suspend after the lid is closed; it is just delayed. If you want to change the default behaviour, then go to
Specifies a period of time after system startup or system
resume in which systemd will hold off on reacting to lid
events. This is required for the system to properly detect
any hotplugged devices so systemd can ignore lid events if
external monitors, or docks, are connected. If set to 0,
systemd will always react immediately, possibly before the
kernel fully probed all hotplugged devices. This is safe, as
long as you do not care for systemd to account for devices
that have been plugged or unplugged while the system was off.
Defaults to 30s.