Does Quick Look work with a small test file?
Create a file named preview-test.md with # Preview test and one sentence. In Finder, select it and press Space. If you see source text with the # character, the file is still readable; you have not established that a Markdown renderer is active. If no preview appears, confirm Finder sees the file and the extension is really .md rather than .md.txt.
Is the Quick Look extension turned on?
Apple’s Login Items & Extensions settings list installed Quick Look extensions under General → Login Items & Extensions → Quick Look on current macOS versions. A Markdown app may supply one. Enable the intended extension if present, then try the minimal file again. The exact panel can vary by macOS version, so use Apple’s instructions for the machine in front of you.
If more than one app supplies a Markdown preview, record which extensions are enabled and test one change at a time. Do not remove other software or clear system caches just because a generic troubleshooting post says to. If the app vendor documents a specific recovery step, follow that vendor’s instructions for its version.
Record what changed
Note macOS version, app version, file extension, whether you saw source or a blank window, and the setting you changed. These details make a support request actionable. Aero Markdown includes a Markdown Quick Look extension in its native project, but the Mac app is not publicly distributed yet; this guide is not a report of a reproduced Aero installation failure.
For a normal Quick Look walkthrough, see preview Markdown in Finder.