… and also using debug mode, but message didn’t show up.
This means that something is interfering with the AJAX requests, and never receives the “signal” to display the alert. If you have an URL of where this occurs, I’d be happy to take a look and try find the cause.
Same problem here. In the page source I see … Why?
The alert is always written as the page output, but initially hidden. This is to accommodate caching plugins such as WP Super Cache, as well as users that do not have Javascript enabled (an additional stylesheet is loaded within a <noscript>
tag that overrides the "display:none"
bit and hides non-functional items).
For those that do have Javascript enabled, an AJAX call will determine if the alert should be displayed, and move it to the top of the content and remove the "display:none"
.