Toggling to external display using TMM may take two attempts.
Consider the following scenario.
You have Windows Vista.
An external display that was never previously detected by this computer is being connected.
The computer is a laptop.
You attach the monitor and set the Transient Multimon Manager (TMM) to make the external monitor the default monitor but only the internal screen displays.
For more information on this issue, including potential causes, workarounds, and resolutions, see: Microsoft KB Article KB972492.