Thousands of Pages not Indexed

What to do?

[Seattle_SEM]

One of my clients has ~4000 pages, on a PR6 site, which have been sitting there since pre-dominic, waiting to be indexed.

Google has indexed about 1200 of these pages, and they are all cross-linked, but it's not even picking up all the pages that it has links to. I can go view a cache on one of the pages which Google has indexed, and see links to 2-20 pages which it has not indexed.

Has anyone else experienced anything like this?
Any suggestions?

These are all high-quality, unique, non-spammy pages.
Thanks.

[Dolemite]

Take a look at link structure. Is there a correlation between depth and indexing?

Try to get external links to internal pages, especially internal directory/site map type pages that contain lots of internal links.

[antizwerg]

This might just be a rumour, but isn't the number of pages that google will index correlated to the sites page rank?

Maybe you have maxed out already.