Not sure what you mean. Maybe you can try describing your workflow in greater detail.
Have you tried resetting your fcp preferences?
Possible workarounds: make the timeline active by hitting command-3, keep the timeline with a "wider" view (hit option -)
I am doing a 2 camera project. I get it all set up and begin to "switch" using the viewer window. I can cut back and forth, then without notice, the ability to switch stops and the highlighted video source in the timeline deactivates. So, I have to scroll back to the last cut, highlight the video in the time line, again, then hit the space bar and continue. This happens over and over but on an irregular basis. I'll try the 2 suggestions above but the timeline is active when I start.
I'm assuming you're using the mouse to do the "switch." If so, have you tried using the keyboard shortcuts instead? Wonder if it's something in your mouse settings that is causing this. what is your hardware configuration (you might update your profile with this info)?
when you say "highlighted video source" in the timeline, what do you mean? Do you mean that you've selected the clip in the timeline? That's shouldn't be necessary. With the timeline active and nothing selected, select open in the canvas, hit play and then start switching.