A few things are known to restrict the mouse on any given machine: -
Heavy load on either PC
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If either PC is under heavy load, it could restrict switching for the time that they are.
-MP itself:
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Multiplicity settings restrictions
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UAC (User Access Control)
If a UAC window is open on any MP machine; example:, it could play a role in restricting Seamless switching
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However, one should be able to unload MP on the machine the cursor is restricted to to have the mouse return to the Primary:
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