Oh~ you didn't mention clearly on your post so I thought you were saying can't comment in normal "discussion" topic ^^' Hmm Maybe it's the user's Miiverse setting? The 1st option is about "who can comment on your posts?" and the option is: everyone, Wii U friends only, or only users who've played. I'm guessing the user use the either 2 later option, so that's why you can't reply on it.