What version of RoboHelp are you using?
Did you create the folder from inside RoboHelp or did you create it using Windows Explorer?
Have you got Show Empty Folders turned off?
Thank you both. I created the folder from inside RoboHelp (9). I didn't know about the Show Empty Folders option but thanks to you I do now! I ended up contacting Adobe Support and they eventually had me enter the registry and rename the RoboHelp folder to reset things. It worked but updating registries is not my preferred way of managing issues.
So unfortunately we still don't know whether the change Peter suggested would have done the trick.