Hello Minka,
If there are no requests received by your service, App Engine does not create new instances, and reduces the number of instances to a minimum, depending on the settings you have chose in your project. How does your app.yaml look like? You should not publish any confidential information here, such as your project ID. Rather, send such information by means of a private message, generate from the drop-down menu offered by the "reply" button at the top right of the edit window.
How did you verify that instances are running? What are the relevant entries in your app's logs?
What is the output of the gcloud info command, on your local development machine?