What I have found is that the blogs that I am trying to comment on are using a spam plugin. I had the same problem a few months back and had to disable my WP-spamshield plugin.
We actually don't need any sort of spam plugin if our sites are hosted here a Wealthy Affiliate because of the amazing site security we have here. So if you are using this plug in, you may want to disable it.
I also recommend removing any captcha plugins because it isn't needed. But also the captchas often don't work and this in itself could be preventing folks from getting more comments on their site.
Have a great day!