By default, your Pixie turns on its WiFi module once every hour to sync up with the server. The sync-up usually takes less than one minute. For the next 59 minutes the timer will be sleeping and disconnected from the WiFi until the next sync. When the timer is sleeping, it is unaware of the schedule changes, including the manual watering.
If you have to water instantly, you can go to Settings -> Preferences-> "Keep Online" and enable the feature.
You can view more info here: https://netrohome.com/en/shop/user_guides/2
Please let us know if it helps.