This can be for two reasons:
1. Because you are a first time commenter. In this case the comment is held until I approve it. This is to prevent spam. After you are approved the then all your future comments will go up automatically.
2. The spam filter caught it. Spam is detected in a number of ways by the filter: an usual number of links in a post, many small paragraphs, words in capital letters, unusual spacing of paragraphs, strange characters, etc. Since the first of the year the filter has caught almost 214,000 spam posts – so you can see it is an unfortunate necessity. I check spam a couple of times a day to see if any legitimate posts are caught. If you are in the filter than it may take half a day before I can get around to plucking you out.
Please don’t double post if your comment hasn’t appeared and I find double, or triple posting awaiting approval or in the spam filter, I usually will delete them all as I don’t have time to compare them to see if they are different. It is far better to send me an email letting me know that your comment is stuck somewhere and I’ll get to it in the next 5 or 6 hours.