Unable to get deep pages have a PR

Problem getting deep pages indexed in google/yahoo/msn

[sandyk20]

Hello,
I have running vbulletin forum software and it has a spider-friendly version of its threads/topics known as archive.

I have following url for my forums: forums.mydomain.com
Link to archive is as follows: forums.mydomain.com/archive/index.php/

I am unable to get a PR value on archive page link mentioned above.

I have my all my forum threads/topic pages link to archive, but not luck. Around a year back I had a PR 5 on the archive with many sub-pages having PR4 or less. I had major content indexed via the arhives, but now it has been wiped off. (no idea about this)

Can someone help/guide me to get a PR for the archive page and getting more content indexed from archive pages.

thank you.