It is not that effective because the free content is not what your readers really need and want. It has to be value-added content that is related to the blog post or article.
1. Look For Blog Post With The Most Online Traffic
To identify the page that receives massive traffic, log into Google Analytics. Click on Behavior, then click Site Content and then click Landing Pages.
Similarly you can log into the StatCounter and look for Popular Pages.
2. Write Blog Post That Adds Value To That Blog Post
Giving away relevant and useful content is an important element to attract more subscribers to sign up their email address to you.
It can be an informative tutorial, infographic, useful tips which you compiled from various sources, an interview with an expert, etc.
3. Create Professional Looking Content
When people have taken the initiative and interest to sign up, you need to take the trouble to deliver your best stuff to them. Present your content in a professional manner.
Don't just trash out a text-filled pages.
Take your time to create your content with a great layout that spells valuable, quality and important.
If you cannot do it well, seek help from Elance.com or Fiverr.com.
4. Place Call-to-Action Link To Blog Post
For effective results, you should box up the call-to-action link in a colored box. Visually it will get the readers attention. Make it bolder.
You should place it right after the blog post and not after the comment section. When they click on the link a lead-pages box appears, asking them to opt-in.
5. Present Attractive Looking Opt-in Box Design
Again spend time or even some money to create an impressively inviting opt-in banner (with good layout design and colors) to entice the readers to sign up.
If you are not a good designer, get one for just five dollar at Fiverr or at Elance to do it for you.
My advice is visit some of the more popular websites to see how others create their opt-in banner designs.
Take some time to do as I have mentioned above and you should get more email subscribers right away.