Begin with a reproducible sample
Use a local file you are entitled to inspect and retain the original unchanged. Record the filename privately, the intended player, the operating system and what actually fails. A file that opens with no sound raises a different question from one that cannot be identified at all. Do not start by converting the only copy: conversion can alter the evidence needed to understand the original symptom.
Ask for structural information
The official ffprobe tool reports container and stream information. A compact starting command is ffprobe -v error -show_format -show_streams -of json "sample.mp4". Substitute your real local filename and use the installed version’s documentation. This is an inspection command, not a remuxing or transcoding recipe. Its report can identify separate video, audio and subtitle streams rather than treating the extension as a complete compatibility description.
Turn the report into a comparison
Consider a hypothetical file with one HEVC video stream and two different audio streams. If the player displays the picture but the selected audio fails, preserve each audio codec, channel count and selected-track detail. Do not conclude that HEVC is the cause merely because it appears prominently in the report. Compare the relevant track with the player and device documentation, and keep an unknown or missing field labelled unknown.
Share only useful fields
For support, provide the relevant format, track properties and exact error, together with the tool version. Review the report for private paths, titles, tags or embedded URLs before sharing it. A successful probe establishes that this tool could inspect the sample; it does not prove that a television decoder, protected-media licence or UK IPTV account can play it. The next action should follow the observed mismatch, not a generic conversion preset.