It probably isn't stuck. You just need to "force" it to the other monitor. You can change a setting to make it like you expect.
Open the CompizConfig Settings Manager (or install it if you don't have it) -> go to Ubuntu Unity Plugin (again, you may need to install this if you don't have it) -> Experimental Tab. At the bottom uncheck Launcher Capture Mouse.
It probably isn't stuck. You just need to "force" it to the other monitor. You can change a setting to make it like you expect.
Open the CompizConfig Settings Manager (or install it if you don't have it) -> go to Ubuntu Unity Plugin (again, you may need to install this if you don't have it) -> Experimental Tab. At the bottom uncheck Launcher Capture Mouse.