Support article here > iCloud: Resetting your Photo Stream
If photostream only works when your device is connected to wifi
On your iPhone tap Settings > iCloud.
Make sure Photo Stream is switched ON.
or connect the phone to wifi, but even that didn't work
If photos still don't appear on the iPhone with a Wi-Fi connection, tap Settings / General / Reset / Reset Network Settings.
Restart the iPhone.
Hold the On/Off Sleep/Wake button down until the red slider appears. Slide your finger across the slider to turn off iPhone. To turn iPhone back on, press and hold the On/Off Sleep/Wake button until the Apple logo appears.