I created my Robohelp file last week with a lot of topics. Tried to open the helpfile this morning and noticed some of the created topics were missing. Checked 'Broken links' in the Project Manager pod. Found no broken links. I can see the topics in the Robohelp project folder.
Finally after wasting a significant amount of time, I tried out something that worked!!
File > Import > HTML/XHTML File and select the html file (the missing topic) from the folder where the project is located.
Didn't find any help/support on this issue. Hence adding this useful post.