Stuck at PR3--- why?

I have 4 different websites that I send various PR and types of backlinks to. I have inner pages, home pages, sales copies..... yet I cant get anything over a PR3.

Not like I am concerned as PR means nothing to me right now but I would certainly like to know what the heck I am doing wrong.

I continually backlink using various methods yet after each google PR update I cant get any page over pr3.


Yet something like "backlinksforum" is a 2 month old domain and is already a PR4??

Trying to learn from my mistakes....


thanks