Ask what the label actually identifies
A container groups media and related information, while codecs concern the representation of individual streams. RFC 6381 addresses media types that can contain different formats and the need to identify their codecs and profiles. This is why two files with the same broad extension can behave differently. A player opening one sample does not establish that every other file sharing that extension has identical requirements.
Collect the relevant details
When a legitimate player exposes media information, record the container, video codec, audio codec and any profile details it reports. Keep the original error message as well. These observations are more useful than changing a filename to make it resemble another successful sample. Do not assume that a decoder mentioned in a device advert covers every profile, combination or output arrangement that can appear within a container.
Test picture and audio independently
A file can reveal different results for its components: the picture may appear while sound is absent, or one subtitle type may be unavailable. Note those distinctions instead of reducing everything to a claim that the file does not work. Compare authorised samples deliberately and retain the source of each result. A familiar container name should not cause a valid audio-path problem to be overlooked.
Avoid universal compatibility claims
The standards vocabulary helps explain what needs checking, but actual support belongs to the player, device, software version and content combination. It does not follow from one logo or extension. If a source is protected or restricted, format support also does not grant permission to open it. Ask a specific compatibility question with the observed media details rather than requesting an unknown all-format installer that promises to activate everything.
Questions, answered
Can changing .mkv to .mp4 make an unsupported codec work?
A filename change does not re-encode the media or create a valid different container. Identify the actual contained formats and use a supported, authorised playback route instead of treating an extension as a conversion tool.