Problem with images disappearing from project

I have inserted a number of images into my Robohelp 9 project topics using the Insert >> Image function.  Typically the images were stored on my hard drive on my pc, but some I pulled from a shared network drive that were prepared by other people.  They seemed to load into the project OK.  I had them grouped in a about 5 different robohelp folders to keep them organized so after I would insert, complete my topic and save.  I would move the images to their respective folder.  This was fine for a while,  I had no problems generating printed .pdf documentation, making changes to my topics, etc.  But now the images keep disappearing from the Robohelp folder structure and the printed documentation.  I can see the images when I open up and view the topics.  I can also see them when I look out onto the drive on the computer.  However, they do not show up in the Robohelp  Project Manager and are not included in my printed output.  I keep re-inserting them, but they continue to disappear.