Green Screen not triggering when I click it

Updating software (app and operating system) to make everything compatible often will resolve misbehaviors. You didn't mention what iMovie version and operating system that you are using.

Here are some other diagnostic procedures that you can try to help isolate the problem:

Try booting up in Safe Mode and, after everything loads,

immediately reboot in normal mode. Don't try to open iMovie in Safe Mode

because it will not do so. After rebooting in normal mode, see if the green

screen works.

If Safe Mode does not solve it, try redownloading iMovie. To

update/redownload iMovie 10, drag your iMovie 10 app from the Applications

folder into the trash, but do not empty the trash. Sign in to the app store and

open your purchases folder by clicking on your name at the bottom of the app

store side bar. You will see your purchases displayed, including iMovie 10.

Redownload/update iMovie from there. If not successful, drag your present iMovie

app out of the trash and return it to the Applications folder. 

If still not working, and if you haven't already tested with

another project, try creating a new project and see if you can share out with

different media and also with the same media that you are working with

now. 

Another thing that you can try is to create a new user account

(System Preferences/Users and Groups) and see if iMovie will work properly in

the new user account. 

All else failing, try a System Management Controller reset:  

https://support.apple.com/en-us/HT201295

-- Rich

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That's not supposed to be happening, as you have said.

It could be a corrupt preference that is causing it. Try closing iMovie and then opening iMovie while holding down the Option and Command keys and selecting to delete preferences in the box that appears. iMovie will open in a new library. Reopen your old library to get back to your projects. Now see if the green screen feature works properly.

-- Rich

I would expect that you would have at least a few purchases. Did you click on your name at the bottom of the app store side bar to open your account? That would reveal your purchases.

No, do not empty the trash. Drag your iMovie app out of the trash and replace it in the Applications folder. If you empty the trash you will delete the app. You want to keep the app.

-- Rich

It occurs to me that you may not be signing in with the same Apple I.D. that you used when you registered your computer and/or used to purchase iMovie. You did acquire iMovie at some point because you have it on your computer now. So it should show in your purchases folder provided that you are signing in with the same Apple I.D. that you used to acquire it.

-- Rich

Hmmmm. That's odd. The transitions display for me when I have No Theme selected. I am running iMovie 10.1.12 on Mojave. I am not able to replicate your findings of no transitions display.

Try these procedures and see if that cures it:

Try opening iMovie while holding down the Option and Command keys and selecting to delete preferences in the box that appears. iMovie will open in a new library. Reopen your old library to get back to your projects.

If still no luck, try redownloading iMovie. To update/redownload iMovie 10, drag your iMovie 10 app from the Applications folder into the trash, but do not empty the trash. Sign in to the app store and open your purchases folder by clicking on your name at the bottom of the app store side bar. You will see your purchases displayed, including iMovie 10. Redownload/update iMovie from there. If not successful, drag your present iMovie app out of the trash and return it to the Applications folder.

-- Rich

Any luck with this yet?

-- Rich