This is the reason behind webmasters always keeping one eye on Google PageRank update. Google doesn't do it on regular basis. Neither there is a fixed time for this. There is no one who can tell exactly when next Google PageRank update will take place. Google keeps few things hidden and you never get the idea behind why Google does this
Google PageRank update schedule for 2013,
|First PR Update||January 31 - February 1st Week|
|Second PR Update||June 28 - July 1st Week|
|Third PR Update||September 28 - October 1st Week or Earlier|
|Fourth PR Update||December 15 - January 1st Week or Later|
These are just roughly calculated by me. The PageRank Updates might take place between these dates or earlier or later. We can’t say anything right now because Google never does things on time!
Why your Pagerank is not increasing ?
The first update isn’t far ahead, so start Building links. Cheers :)