iMovie issue

For some reason I can't drag any of my photos from my new project into my timeline. All the photos have the camera symbol in the corner but when I try to drag them down onto the timeline, a circle with a line through it appears on each one. The photos come from different sources but include photos taken on my iPhone. It's odd as I've recently produced a couple of iMovies without any problems. I have also had a warning box come up with "You are combining audio & video clips outside the primary storyline" - not sure what that is about! Any help will be gratefully received!