You have to invoke the rendering process either by selecting a clip (or clips) so that they become highlighted in yellow, and then from the menu bar choose "Render Selection". You can also click in the timeline so it becomes active, and then from the menu bar choose "Render All" to render the entire project timeline.
Alternatively, if you set "Background Render" to ON in the FCPX preferences (under the playback tab), then FCPX will begin rendering automatically after a preset delay of inactivity. This is not generally recommended as larger projects may incur corrupt render files as FCPX re-renders any changes made on the fly. I suggest leaving this option OFF, but just wanted you to be aware of the other way FCPX will render clips.
If the above doesn't solve your problem, try closing FCPX and re-launching it while holding down the OPTION and COMMAND keys to bring up the delete preferences dialogue - then choose ALL to delete all preference files. When FCPX starts, go back into preferences and turn Background Render OFF, as clearing preferences results in it automatically being turned ON. Report back if you're still having problems.
I did indeed have background render on, which is why I was confused. However, due to your suggestions and others I think I am going to turn background render off. I will report back with how that goes while working on my current project with render set to OFF.
I'm not a 100% on this but I think with background render on, FCP only renders what is necessary to have full fps playback. Certain codecs and/or effects can be processed in realtime so no need to render.
I agree though to have background off and render using the 'render selection' where necessary. Avoids a lot of processing because otherwise every little change would trigger a new render.
I am having this exact problem. I have one clip that just refuses to render even when I have selected it and used the render selection method. It is the only clip that is doing this and when I playback it judders. I am thinking of perhaps erasing it and restoring it. What do you advise. I am on FCP 10.6. Background render is off. Thanks in advance. Dawn