The most common reason is that the host is offline or hibernates.
Other times, the package may be waiting for the Windows Installer database, it will wait for up to two hours, before abandoning the update. The update will not be retried until the next regular or manual inspection.
In rare cases an installer may hang, this can be due to local configuration issues, conflicts or unexpected dialogues (which usually will be invisible to the user).
The package will time out after 59 minutes.
The update will not be retried until the next regular or manual inspection.
Also, only one package task can run, despite the Applying state in Package Activities, the other package tasks will wait for the first package to complete or exit due to one of the above mentioned timeouts.
The order in Package Activity is not indicative of the order in which multiple packages are attempted.
Note: In rare cases the Windows Installer database is locked for a very long time, this may be due to other installers running including Windows Updates. In some cases a restart of the host may be required to free the Windows Installer database again.