To get more people to read your feed, you should try to find what most interests your readers. Since my German is horrible (I have not used it regularly in the past 10 years), it is difficult for me to comment specifically on your blog.

You can use the SEO Booster PRO plugin to export a list of keywords that your visitors use to find your blog, and focus on those most used, and add more content related to those keywords. That will bring in more targeted visitors, and increase your chances of gaining more RSS readers.

Also, remember to engage in conversation with your readers. I try to always comment/respond to any comments made on mywordpress.com for instance, and that will bring commenters back, and eventually even make them subscribe to my RSS feed.

(You should take a look at the Subscribe To Comments – plugin, where people can sign up to automatically get an e-mail if anyone responds to their comments, like you will when I enter this response).

I hope these observations can help you in any way.