I suppose the pagerank changes everyday, its just that the visible pagerank that you see in the toolbar is changed once in three months.. like every one has already suggested that you continue with your link building.. see to it that its not too aggressive though
PR is taken as quality of content you provide to your users . If you have good information which is unique and not known by others than keep posting such posts . Your PR will be up next time . But you need to build few good links also from niche related to your blog .
there is no exact timetable. Google has had extended periods as long as 160 days without an update. They have had periods as short as one month. On average--- the frequency is 4x per year