If you don't know proper on-page SEO then your rankings will drop each time you create a new post on your site because the link juice is now getting spread to many pages. You need to come up with a good interlinking strategy for your website so that you can keep the juice flowing within the website without wasting it.
Do some research about silo seo structure, here's a thread that might help:
http://www.webmastersun.com/threads/8507-What-is-SILO-In-SEO
Honestly, posting new content frequently doesn't help improve rankings. It's a good strategy to keep your blog readers and social media followers engaged. Frequently updating a blog is a good traffic generation strategy but can mess up your rankings if you don't know proper on-page seo.
So to get back your rankings you'll need to create some new backlinks, not just any weak backlinks though. You'll need some strong backlinks from high authority sites that also have strong backlinks linking to them.
Maybe there's also too much outbound links on your site, these are links pointing to other websites. If that's the case then you're sending some of your link juice to external websites which leaves your site with less juice to be able to rank high.
So what you need to do is to stop linking to other websites, do some proper on-page seo then get more strong backlinks and your rankings will definitely improve again.
