Make sure you moderate the first post or the first two posts.....that will show you if any new ones are getting through.
I found a way that works for me. Formulate a CAPTCHA question that asks about something in the future. Spambots can't speculate about the future. They have to use existing data and there is no data for the future. For example, during the elections I asked "who will win the presidential elections?" There were only two choices that any human could guess at but spambots could not. After the election I changed it to "what holiday will you celebrate in December" and gave a list of choices. So far so good. I think the key word is "will." Bots don't have a will, they're programs. I've been spam free for over a year since I started using this method. I got the idea from Kier who said "try to think like a bot when you formulate your CAPTCHA question." Bingo!