Mouse is stuck on the Secondary

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. 

Multiplicity settings restrictions

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UAC (User Access Control)

If a UAC window is open on any MP machine, it could play a role in restricting Seamless switching

However, one should be able to unload MP on the machine the cursor is restricted 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:

Open