Search crawler question?

I have a site that Yahoo and Google have found.  Good.  The index page
has links to 4 other pages.  The search engines find 3 of the pages
.... but.... the one that I care about..... it doesn't find.  I've run
the W3C checker on all the pages and they are all OK.  What possible
reason could there be for the 4th page to be ignored.  Note:  the pages
are linked to via anchors, and are generated by perl scripts.

TIA
Will