iMovie stopped recognizing my green screen halfway through project

My green screen is a bright yellowish green. For the first half of my project, it sometimes wouldn't select it but with some fiddling I could get it. Now when I choose green screen it selects something else. Even when I crop so there's nothing at all green or blue visible, it still doesn't select it. When I use the tool to click the greenscreen it disappears but something else is always missing. One person's jeans. So annoying because it worked an hour ago! Any suggestions? I rebooted several times, deleted preferences, no dice.