Expert search works on the skills that are on people's profiles. If a search finds nobody for a skill, one of these is usually the reason.
The skill is not on anyone's profile yet. In Strict Compliance mode, a derived skill sits in the Suggested tab until the person accepts it, and suggestions do not count in expert search or in skills coverage. In Maximum Velocity mode, the scan may simply not have reached that person's work yet. This is the most common cause in the first weeks after installation.
The work behind the skill is out of scope. The app reads resolved Jira issues outside Jira Service Management, and Confluence pages and live docs. Someone whose expertise lives in Confluence comments, in open issues, in support tickets, or outside Atlassian produces no skills for it. See Which content does the app read?.
The skill is on the blocked list. Anyone can block a skill for themselves, and then it never comes back until they unblock it.
The name does not match. Skill names are matched after normalization, so C# and C Sharp count as one skill. Real synonyms do not: P&L and Profit and Loss stay two separate skills, and a search for one does not find people who recorded the other. Try the other wording, or a broader term.
The search extracted a different skill than you expected. The result list shows the skills the app took from your text, including the ones it found nobody for. Check that line first. Entering a comma-separated list of skill names gives you more control than a sentence.