If you open the Symantec Management Agent and the Task Status tab shows that the task is running, then it is firing off the task. The server may have an outdated task status sometimes. At times it can be because you haven't used the refresh buttons to refresh the status. At others, it can be that a task hasn't reported any status. It's given the computer a task, the computer is still thinking about the task, there is nothing new to report.
The cause is likely that the .exe for the command line is misformed or the credentials used to execute the .exe are not an administrator or not running in the correct session. For example, try changing it from Symantec Management Agent credentials to 'Currently logged on user.' Alternatively, if you're running it silently, start out by just running it with full UI, no switches.
If you run it as a different user, is it successful? If not, what if you drop the switches and just install the program normally? Does the UI appear?