Having a 'subscribe to posts' button is usually not enough to attract subscribers. Some people are so used to seeing subscribe buttons, they don't even really notice them anymore. Others don't understand what clicking your subscribe button really means, so they won't click. The easiest way to gain subscribers is to invite them. Write an invitation next to your subscribe button and at the end of each of your blog posts. Something simple like "If you like my blog, just click the subscribe button at the top of the page and you'll receive an email each time I post something new" will work just fine.