Google Caching

Its weird that I have a news section on about all of my websites that I use wordpress for. One or two of the sites as soon as I publish the post it is cached by Google and on others it take anywhere from 1 day to 5 days for Google to cache it.

Anyone have any ideas why this varies so much? Its funny because one of the sites that it caches almost immediatly I rarely even update. I may write 1 or 2 posts a month and my main site which gets the most traffic and has the highest PR it seems to take forever.

Anyone have any ideas on how to improve the time it takes big G to cach a post?

I've seen this before and to be honest the answers I got didn't seem to add up

I was told get more authority and social media use, i.e more links and shares/tweets etc but the I have seen lesser sites get cached quicker than more authority site, god knows why

I'd update the ping list, get a few more links to catergories and share on facebook and twitter etc but if you do find the answer let me know :-)