Not off the top of my head. I have found that sometimes the captcha is slow at generating and being returned, but as long as you wait a few seconds it always appears. If you wait, does it eventually get generated?
I find it strange that it always works on the server, but not always off the server. Are you sure there are no content filters/proxies between the server and the client that could be screwing things up? Also, have you tried different browsers to see if it
happens in all browsers, or only a certain browser?