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- Photostream only syncs photos over Wi-Fi, make sure you are connected to Wi-Fi to begin with.
- Photostream only syncs when the camera app is closed, ensure it is closed.
- Photostream only syncs when the battery is above 20%, try recharging the device
Try disabling photo stream on your device (settings > iCloud), restarting your device and then re-enabling photo stream.
Note: disabling photostream and re-enabling it will result in any photos that are currently in the photostream album on your device, that are older than 30 days old, not being added back.