A year ago, I noted that Language Log was getting about 4000 spam comments per day ("A million (spam) comments", 9/2/2011). Recently, this number has been substantially lower — just 771 in the past 24 hours, for example.
But during the past year, the spam tide rose before it fell. We logged our millionth spam comment at some point early in the morning of September 1, 2011; today, a year later, our spam filters have caught 3,277,574, or an average of (3277574-1000000)/365 = 6240 per day over the intervening time.
Whether the spam filter's daily harvest is 500 or 10,000, it's too many for me to check the whole list for false positives. So if your comment doesn't appear, and it wasn't offensive and/or devoid of relevant content, there's a good chance that you somehow got caught in the spam filter.