Read an awesome article on Zemalf.com last week about comments being blocked by akismet and what you need to do to find out how and if this is happening to you.
I am not going to rewrite the article and exact steps shown in that blog since I want you to read the original article, but will instead give you high level overview and a summary of what you will find in the article and how to tell.
When you leave a comment on a blog and you do not see your comment post right away, it can be held for moderation or immediately blocked by an anti-spam plug-in. Usually you will see a message saying your comment is held for moderation but if you don’t see any message at all, then you can check to see if you are being blocked by the Akismet anti-spam plugin which is one of the most popular ones in use. Note, this won’t work if it is the WP-SpamFree plug-in that is blocking your comment and you really don’t know which plug-in the blogger is using.
The article lists a site that uses Akismet and allows all comments, you test your comment on this site, if it doesn’t show up then you know Akismet has marked you as spam.
Then you can find the contact information for Akismet so you can email them and have them investigate and remove you from their spam list.
So if you find that you are laving comments on others blogs that aren’t showing up, I highly recommend you read the article How to stop your blog comments from being blocked by Akismet on Zemalf.com and hopefully it will solve your problem.