Halfdeck - 4:27 am on May 29, 2007 (gmt 0)

If a url's PageRank falls too low, it will fall out of the main index. If your site is largely supplemental, it means 1) not enough quality inbound links to your site 2) you have too many pages 3) you link out too much 4) Google may think your IBLs are artificial 5) Cannonical issues are causing PageRank to split.