why google cached does not update all the time(15 days)

I am updating new products to my website, yesterday I changed the title, and google update my title today, but it still doesn't update my google cached about 15 days. So why? What is the problem?

With the billions pf pages stored on their servers you should expect delays because they factor their algorithm and rankings based on cached pages data so don't expect their caching of your website to be daily or even 2-3 days job. But if you have a website that gets tons of genuine fresh content every day google will index/cache/refresh cached pages on your site faster.

You can ad instructions for their crawlers in your page source, like


<meta name="revisit-after" content="1 day">