You have to use sufficient brake pressure ...here's a description of how it works

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INTELLIGENT STOP/START

The Intelligent stop/start system is designed to
improve fuel efficiency and is automatically
activated when the ignition is turned on. Unless
it is required to support other vehicle systems,
the engine will turn off if the vehicle is stopped,
for example, at traffic lights. When the brake
pedal is released and a drive gear is selected,
the engine will restart automatically.
During operation of the Intelligent stop/start
system, a warning lamp will illuminate. See 45,
INTELLIGENT STOP/START (GREEN).

Always disable intelligent stop/start
before wading.
To activate an automatic engine stop:
• Stop the vehicle from a speed greater than
4 km/h (2.5 mph) and apply sufficient brake
pressure to make sure the vehicle is
stationary.
• Alternatively, while stationary, select Neutral
(N).
To activate an automatic engine restart, release
the brake pedal with Drive (D) or Sport (S)
selected.

The engine will also restart if one of the
following occurs:
• Intelligent stop/start is deactivated.
• The accelerator pedal is pressed.
• A shift paddle is used to select a gear.
• The steering wheel is turned.
• Reverse (R) is selected.
• Climate control system demand increases.
• The vehicle’s speed exceeds approximately
1 km/h (0.5 mph).
• Battery charge becomes low.
• Brake vacuum has been reduced (e.g., using
the brake pedal repeatedly with the engine
off).

The following conditions will prevent an
automatic engine stop:
• A shift paddle has been used to select a
gear.
• The external temperature is less than
approximately 0°C (32°F).
• The external temperature is more than
approximately 40°C (104°F).
• The engine has not reached operating
temperature.
• The driver’s seat belt is not fastened.
• The Climate control system demand
requires the engine to be running (for
example, in Defrost mode).
• The battery charge is low.
• The bonnet is opened.
• Intelligent stop/start is deactivated.

Note: If, during an automatic engine stop, the
driver's seat belt is removed and the brake pedal
is released, the vehicle's engine will turn off and
Park (P) will engage.

DEACTIVATING INTELLIGENT STOP/
START
To switch the system off, press the ECO switch.
See 298, DRIVER CONTROLS.
Note: The engine will restart automatically if the
switch is pressed while an automatic engine
stop is in progress.
To confirm that the system is off, the message
ECO Stop/Start Off is momentarily displayed in
the Message centre.
Note: The Intelligent stop/start system will
automatically reactivate the next time the
ignition is turned on.
If the ECO switch is pressed while there is a
fault, the message Eco Stop/Start Not Available
is displayed.

DRIVER EXIT
To prevent the vehicle from being inadvertently
left in a driveable condition, the vehicle will
detect when a driver is not present, and
automatically turn off the ignition.
If Drive (D) or Sport (S) is selected, the Driver
exit feature will turn the vehicle's ignition off if
the following conditions exist:
• The driver's seat belt is unbuckled.
• The brake pedal is released.
If Neutral (N) or Park (P) is selected, the Driver
exit feature will turn the vehicle's ignition off if
the driver's seat belt is unbuckled.
Once the vehicle's ignition has been turned off,
the vehicle can be locked. For more information
see 9, SINGLE LOCKING