Topics Randomly Appear as Missing Files

I've been using RoboHelp 6 with RoboSource Control for 2 months now. There are two of us that edit one project typically without any problems however recently a red X has appeared through a few topics indicating (I believe) that the topic is missing. Since the topic is still in the project folder on my computer, I try to import it to fix the problem but that doesn't work. Any ideas?