Pagerank that you can see in the google toolbar is merely a snapshot in time. They take that snapshot usually around 4 times a year, though at times it was nearly a year, and other times it was almost every month.
Whatever you do that may cause pagerank to increase will only be visible when Google does that snapshot.
Internally, Google updates pagerank far more often. You could see a PR0 and in reality it could be PR6.
For instance, if you post an article through a massive high PR article network, and your article is on every homepage, and Google takes their snapshot, your site could show as a PR5. Then, next week ... all your homepage posts get moved to the 2nd page and your PR drops down to PR2 (because all those high PR homepage backlinks suddenly became low PR 2nd page backlinks), but you wouldn't see that you dropped down to PR2 until the next visible Google PR update.