Note the time playback stalls, how long it stalls, and whether it resumes or fails. Compare one other video on the same service and, if practical, the same video on another device or network.
Include app or browser version, selected quality, approximate region, and local time. A single damaged video, a slow local connection, and a wider service incident need different fixes. Do not share private watch URLs or account details.
Does the buffering affect every video or one title?