Pixie is connected to the cloud service through WiFi. Because distanced WiFi connection consumes much power, Pixie will go to sleep mode to save power after 5 minutes when it is manually powered on.
If you have to water instantly, you can go to Settings -> Timer-> "Keep Online" and enable the feature. Note the timer may start to keep online the next day and battery consumption will be faster when the timer is in this mode. In the regular mode the timer is sleeping you will need to wait a bit. In the "Keep Online" mode yes you can start it instantly. Here is an article about this: https://www.netrohome.com/shop/user_guides/2