Afaict, the plugin doesn’t support github issue comments, despite issue_comment being listed as a subscribed webhook event in the plugin docs. The plugin is architecturally built only for commits and PRs. Regular GitHub Issues are not the same as PRs and will silently fail.
There is an open feature request for this you would want to add your vote to:
But if you have the budget, asking in Marketplace is probably the best direct route available.