A few things are known to restrict the mouse on any given machine:
-Heavy load on either PC:
If either PC is under heavy load, it could restrict switching for the time that they are.
-MP itself:
-UAC:
If a UAC window is open on any MP machine; example:
However, one should be able to unload MP on the machine the cursor is restricted to to have the mouse return to the Primary:
If you want to shut off UAC prompts, go to your start menu and type UAC. Click Change User Account Control Settings. A window will prompt you to lower or raise the level of notifications. Select the lowest option and click ok.
Example:
