MauriceW: I haven't had that problem myself, but your web server might be behind a reverse proxy, which makes it look like you (and everyone else) come from a single IP address. In Wordfence's settings, near the top of the page, there is a drop-down labeled "How does Wordfence get remote IPs" -- if you choose the "X-Real-IP" or "X-Forwarded-For" option, it might get it working.
As far as I know, this version doesn't handle that part differently though, so it might just be a coincidence that your site is under attack while you're trying to use it. (Or your host recently changed their setup.)
Wordfence staff: It would be neat if the options page would show you what was in those headers, and/or what it thinks your IP is!