This is something I ran into lately on one of the blogs I manage. People were not able to leave comment because they could not see the word verification text box where they would normally type down what it says on the "captcha" image. See Image for example:
So how do you fix this? Well I did some searching and found a thread on the google help forums titled Word Verification: form box not rendering properly, which helped point out a solution.
The word verification process is displayed in an iframe whose height is limited to 275 pixels which cuts off the bottom of the comment box, thus not allowing commenters to type down the captcha image. To fix this, go to your edit html tab under your affected blog and be sure to download your template as-is to back-up your template if all else fails, then check the "Expand Widget Templates" box and search for comment-editor in your template. It will most likely be found within an iframe. You will find a value in this iframe that reads height='275', or height=275, change the height value to 400. Preview your template, if no errors occur, save your template and your done! Your verification text box form should now appear for your visitors and allow them to leave comments again.