FindSiteMapNodeFromKey doesn't seem to get key for certa

FindSiteMapNodeFromKey is throwing error when I try to create/edit certain pages. It seems the key for those pages are missing. 
Message : Value cannot be null.
Parameter name: key

The error occurs for some pages that are created under a grouppage and these are not shown in navigation. The mysterious thing is that error doesn't occur for every page under this group.

Can anyone pls throw some light as what may be the cause. It has been several days and several posts and no one has answered yet.