Premiere Elements 11 project stopped allowing new slides to be added

I've got 58 slides and a 2 minute video clip in my project. I've been adding a slide and narrating it one at a time. Total time on the rendered project is 53 minutes 30 seconds. I have 6 more slides to complete the project, but now it won't accept a new slide! The last one I tried to drop onto the timeline looks like a ghost and I can't move it, delete it or anything! I did another render and played it back. Everything is fine up to this point. How can I fix it so I can finish this up? This is very important. I need to get this thing done.