When you stream a movie over WiFi or 3G/4G your device will adjust the video quality if your network speed cannot download content fast enough. This prevents buffering.
One work around is to not force start your content. For example, if you are using the YouTube app, when you have a video begin to load try waiting for the video to begin rather than press the play button. When the device detects that enough of the content is loaded to where your network speed will not cause buffering, it will automatically begin playing.