ECO Start/stop not always working

New Aveo owner here :-) 2013 model CHEVROLET AVEO 1.2 5D petrol.

When pressing the "ECO" mode my aveo is supposed to use the Start/Stop function, i.e. automatically turning the engine on and off for example when waiting at traffic lights.

However, it only seems to work after I've been driving for about an hour. Before it never works.

Is this a normal/expected behavior? What could be the cause of this?

Hope someone can help.

I found out myself. I thought the start/stop would always work, but of course it's depending on several factors, such as: outside temperature, amount of gasoline, level of battery.