PR5 on HP, so backlinks don't count for deep pages

My home pages (7 different subdomains) are all ranking at PR5 (had a 6 briefly, but lost it). It is 2 levels deep from there to my product pages. So, all of my Category pages are showing a PR4, but the product pages below that are a PR3 or PR2.

It took me awhile to figure out why backlinks were not showing from my product pages (use a mod rewrite to serve shopping cart, so hard to tell on the toolbar). Anyways I discovered that a few product pages were counting for backlinks, and turns out those are ones that have been featured on the home page (so they got a PR4).

Does anyone have any suggestions on getting all my product pages up to a PR4 (I have probably 2-3000 product pages on each subdomain).

Here is what I know
1) Figure out a way to get my home pages to PR6 via outside links coming in could do it.

But besides that I dont know. Even if i had every page listed on a site map, it would still be 2 clicks away, thus dropping from 5 to 4 to 3.

The good/bad of the industry this site is in is that basically there are only a couple people with PR6 and nobody with PR7, so there is not much to go off of in the way of industry links I can make work to my advantage.

Getting all my product pages to a PR4 would dump a TON more backlinks for many of my Category pages and keywords they are optimized for.

well I think you must be a low PR5, since you are experiencing such a drop off. I think that maybe a couple dozen back links more and your PR will improve enough to bring up the PR3's to PR4's

You need a few PR5's or above straight into the subdomain for your product pages...

My guess is that my main home page is a high PR5, but my subdomain home pages are low PR5's so 2 levels below a subdomain has that drop....