Make sure that you have entered the correct keys in K2's configuration and you are not using Joomla!'s reCaptcha as well.

Finally make sure that your site does not have any JavaScript errors.

I'm having issues with the recaptcha piece of the commenting system not accepting entries. Does this post mean if Joomla's Captcha/Recaptcha plugin is enabled, this is the cause?

Also, this site runs PHP v 5.6 with Joomla 3.4.8

The error when trying to comment reads:
The words you typed did not match the ones displayed. Please try again.

Do not double post.
Your issue will not be solved any sooner.

Well, this seems a bit odd, but it worked. I reset the Captcha/Recaptcha plugin to version 1, leaving the codes as they were for version 2. Tested Comments section again and the captcha is now working.

Hope this helps someone else out there.

Hello,
I am contacting you after months of trying to manage this section by myself. I am not a computer geek, but I know the basics.
I have a new CSS template and the 3.4.8 Joomla version. I'm stuck between Category Blog in the Content and K2 Category Blog, I don't see the solution to the best blogging option. It just doesn't publish as I want it. Should I install another Joomla extension? I also want to disable category filtring, if possible?
Thank you in advance.

Natasha do not double post.
Double posting will not get your issue solved any faster, it will only lead to a suspended account.