It is working like a clock. As it is intended. This command is a developer tool to be used in testing the client. It is not designed for your pleasure. If you tail the log file, you can see the coordinates and direction the wandering horde is spawned.
If you want to place a crowd of zombies in front of you, use the tool in the creative menu or the se command.
I'm talking about the fact that a wundering horde does not spawn every time after using the command in the console. Often there are misfires. Does this have to work that way?