I remember in the past whenever I use to comment on someone's YouTube video, I often got one or more replies, but more recently my comments seem to go unnoticed unless the video had less than 10 comments. Sure enough, the problem lies on how YouTube ranks comments, which I think started happening when YouTube added Google+ integration. YouTube places all the "Top rated" comments first, followed by comments made by what appears to be well-known YouTube publishers or people who probably receive a lot of +1's.
To given an example, I posted a comment on this video, which is about an eBay bought COB LED bulb, basically mentioning about the issue I heard about these melting when they fail.
Once I made the post, YouTube shows it right on top:
When I log out of YouTube, my comment disappears out of sight, that is until I scroll three page-fulls of comments (1920x1200 screen). There were 19 comments above mines (without expanding any top comment lists), all made 1 to 3 days ago and mines is only 18 minutes old in this screenshot:
So the next time anyone posts a comment on a YouTube video, especially on a popular video that has a lot of comments, try opening that YouTube video logged out (e.g. or in a different browser) and find your comment.
I have also had the odd comment that I think was automatically flagged as spam as I could not find any sign of it once I logged out. In fact, that comment I made in the above video was my second attempt as the original comment never showed up while logged out, so in the end I deleted it, reworded it a bit (leaving out the words eBay and Amazon in case they were spam triggers) and reposted it. I didn't have any link in the comment that was censored.
I previously saw the odd innocent comment being falsely flagged as spam on my own videos, which I had to approve. However I never realised the odd comment I was making was being censored unexpectedly until recently and makes me wonder how many comments I made in the past were censored.