[TYPO3] What precludes a page to be indexed?

[lesm]

I know a page must be catched and visited (rendered) in order to be 
indexed.  My question is what else precludes a page to be indexed?  I'm 
asking this because I have some 5% of all pages (out of +6000) in a site 
not indexed and hence those pages can't be searched.  Of course all 
those pages are cached -according to Page Properties- and are already 
displayed in a menu and visited.