The Ikea Kitchen Planner is the software we love to use - and love to hate. I don't have a Mac, but I've had similar problems in the past, and I've noticed that occasional plans posted by others also seem shaky or glitchy.
When it's happened to me, I've just started over. I know that's not what you wanted to hear, but past experience tells me that once I've run into an issue with a particular design, I'm going to continue to have problems with it. If I exit the planner, sign in again and start with a blank slate, the next time it will work.
I'm very glad to know that the world's leaders aren't using the Ikea software when they're toying with the idea of missile launches.