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Try disabling photo stream on your device (settings > iCloud), restarting your device and then re-enabling photo stream.
If this doesn't help you may need to reset photo stream. You can do this at icloud.com by clicking on your name in the top right corner and then the advanced settings in the pop-up dialogue box that appears.
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.