Some scans may take longer to complete, especially if a device is behind a firewall or responds slowly.
While the scan is in progress, any device being scanned will display a question mark icon in front of it. If it seems to be taking too long, please right-click that device and choose Properties. This will show what exactly is being scanned and for how long.
If you let us know what you find, we will be able to look into it further.