Has anybody seen an increase recently in the number of occurences of single pages being filtered out of the SERPs? Anybody have any ideas on the reasons, I'm thinking perhaps a lack of variation in anchor text?
If you haven't varied anchor text much at all, it very likely is the reason for this. When you build unnatural looking links, it is common to have that page penalized. Go back and try to change the anchor text of some of those links and go get a bunch more links using a wide variety of link anchor text. I was recently doing seo for a site with great rankings. One of their internal pages couldn't make any progress in rankings, simply due to the fact that I hadn't used enough anchor text variation. In checking anchor text variation, I believe Google groups plural and singular phrases together as one.