Welcome to iMore! I don't use photo stream. But you might try this:
Quit Photos: Press the Home button twice to see preview screens of the apps you have open. Then find the Photos preview screen and swipe it up and out of preview to quit the application.
Tap Settings, then tap iCloud.
Tap Photos. Tap Photo Sharing to turn it off.
Tap Delete Photos. The photos will be automatically re-added later when you turn this setting back on.
Tap the Home button to exit Settings. Wait about a minute.
Tap Settings, then tap iCloud.
Tap Photos. Tap Photo Sharing (Shared Photo Streams) to turn it back on.
Tap the Home button.
Open Photos. Tap the Shared tab. Tap the name of the shared photo stream.
Test to see if the issue has been resolved
Sent from my ancient but trustworthy iPhone 5