movies constantly buffer

Welcome to the Apple Support Communities.

Take a look at:

• Recommended settings for Wi-Fi routers and access points - Apple Support
• If your iPhone, iPad, or iPod touch won't connect to a Wi-Fi network - Apple Support

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Depending on the type of phone you have, the router you have, and all the devices connected to it, all of these are possible issues why it's buffering. Also, some ISPs will throttle data during peak hours.

Lastly, try resetting the phone and router.