Q: Why Google PageRank is not updating often?
A: Google updates their page rank database every 3 to 4 months and it takes time,  So do not get obsessed by it all the time and refreshing your page 5 times a day which won’t speed it out.
Instead, Think about other features of your site:

ROI (Return on Investment)
Link Building
Website design and overall website performance
Website speed

And i know it is nice to have higher page rank and for corporate users it is still an achievement metrics for website KPI and it’s nice tell …

Google SEO Tips and Tricks:

Google checks for duplicate content on your website.
Google move websites’ ranking down in the SERP when an excessive number of keywords (keyword spamming) are detected.
Google displays META description content in the SERP, so writing good description tag is must for any webpage.