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

Open

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: