Having same problem as other posters re: out of storage space

NOBU won't back up PC, says it's out of storage space, yet storage icon says using only 17.5% of available storage space.  Why is this happening, and what do I do about it?

 

Also I frequently (every week or so) have to reload/download NOBU AGAIN because Home Page is showing my Dell PC and Laptop icons do not show up on my homepage.  Want to know why this is happening also!