Has anything changed on your website in the past 30 days? Google determines when it will crawl your site based on how often content changes.
Please note this is important
: how often Google crawls your site has nothing
to do with rankings. If your site content doesn't change, there's no need for Google to keep coming back to re-crawl. They already have your content in their index, and it will get ranked according to where it should as determined by their ranking algorithm. Even if your content only gets crawled once, and never again, it will still be in the index and subject to the ranking algorithm.
There are two ways to get Google to crawl more often:
1. update your content more often. Keep in mind: updating content for the sake of updating content (just to get Google to crawl more often) and not for the sake of making the content objectively better
is a waste of time. Once again, it won't help you with rankings. The only way to improve your rankings is to make the content better - more useful, more informative.
2. Get more authoritative links pointing to you. The more links that point to you, the more likely Google is to visit your pages just to check and see if anything has changed. The thing is, these can't be just any links; they need to come from authoritative sites. Google isn't going to waste time following crappy links from spammy sites.
The main thing is to determine why you need Google to crawl your content more often. It may be that you don't
need more frequent crawls. One less thing to fret over that way.