Depending on the amount of space taken by the currently active application's menu options, there may be limited space in the menubar for menubar apps, causing them to be hidden.
To fix this:
- Activate an app with minimal menu options (such as Finder) to reveal more menubar apps
- Hold the
⌘ Command
key and click-drag the MultiScreenshot menubar icon to the right for higher priority
- Consider using a menubar management application to help reveal hidden menubar apps