"Markdown not working" in Jira is almost always one of four things. Run down the list in order; the first one that matches is your answer.
If you pasted, that is the cause. Jira Cloud converts Markdown as you type, not on paste. Full explanation and fixes: pasting Markdown into Jira.
They use different syntaxes.
h1. Title,
*bold*, {code}. Markdown syntax will not convert here, and
single asterisks mean bold rather than italic.Both are in the cheat sheet. If you moved from one to the other recently, this is very likely your problem.
Autoformatting only exists in rich text fields: description, comments, and rich text custom fields. Single-line text fields, plain text custom fields, and the summary field are plain by definition and will keep your characters literally.
Some Markdown has no equivalent node in Jira, so there is nothing for the editor to convert to. Commonly missed:
If your syntax is on this list, no app or setting will change it; the target format has nowhere to put it.
If typing the pattern by hand converts but your pasted or automated content does not, then the gap is the input path, not Jira. That is the case a Markdown editor panel is for.
These notes are written by sellerkit. We build a Markdown editor app for Jira (below), which is why we had to work all of this out in the first place.
Everything above this box works whether or not you install anything. Where a workaround is enough, we say so.