Try another item on the same device. If only one file buffers, compare its format, bitrate, tracks, subtitles, and storage location with an item that works.
Open Settings → Status and select the active stream. Record the playback method, source and output bitrate, selected quality, video and audio codecs, subtitle mode, and transcode speed. A transcode can be limited by CPU, hardware acceleration, temporary storage, or a subtitle that must be burned into the picture.
For direct play, compare the media bitrate with the available network path. Test the server over wired Ethernet and remove Wi-Fi, VPN, or remote internet variability where possible.
Lower the selected quality as a diagnostic step. If a lower stream works, inspect Settings → Playback and the remote bitrate limit under Settings → Connectivity before leaving the quality reduced permanently.
If every transcode buffers, copy the active-stream details and the matching Server Console events. If only one device fails, review How clients differ. If playback fails instead of buffering, use Playback failed.