Link pyramid help - web2.0 site is indexed, but no PR?

I've tried my hand at a simple link pyramid. Only a small handful of web2.0 sites have a little PR (all but one came in the Nov 2012 PR update). The rest are PR n/a through several updates, even though the sites do have pages indexed. Is there something else I need to do? Is there not enough content?

you need more back links to your web 2.0

OK, I wasn't sure. I didn't want to create a whole bunch of backlinks to a site if it's never going to be useful...