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Your internal pages, without links from outside, get their PR from your home page. If your home page is a low PR4, and you have a lot of links, then it's very possible to have PR2 internals.
If you get more PR to your home page, then it will flow down to your internals. Deep links could also give PR to your internals. For most sites, deep links are better as then help those pages rankings more. After all, rankings are usually more important than PR.
It would probably benefit you to read a good "page rank" explaination, and possibly play around with a good javascript "page rank calculator". Searches will turn up some good links on those topics.
as already mentioned by ciml, you need deep links to different internal pages. Changing the structure of your site won't help because your structure is already flat.
(Recent thread: How to get a flat Page Rank for entire site [webmasterworld.com])