Each page of a website has PageRank, so it's quite normal for the main page to have a higher PageRank than an internal page.
Visible PageRank, which you see in the Google toolbar, is updated only about four times a year. However, the actual PageRank is updated all the time. So a new page may look as though it has no PageRank when it actually does have some.
For example, the main page of AssociatePrograms.com has a PageRank of 6 but the most recent newsletter which I wrote this week has a visible PageRank of zero.
For a more detailed explanation, click on the "Knowledge Base" tab at the top of the page, and type "pagerank" into the search box.
By the way, what's important is a link FROM a good, similar topic site to your site. You receive a fraction of the available PageRank. The more links on the page, the less PageRank you receive.