We are having this issue on multiple sites or our clients almost 50+, right now we have disabled the captcha plugin as all the clients are running paid ads and was not getting leads due to issues with captcha while filling the form (Gravity Forms).
For instance till yesterday the captcha was not working, we have cleared the cache, and reinstalled the captcha add on with gravity forms.
http://www.stanbennettlaw.com/
now it is showing but within a day or after some time it will stop showing captcha images, insteated it will generate images those images will just lead to 404 Not Found page.
We have discussed this issue with Gravity form's support and they said the issue is with caching plugin so we need to talk WP Super Cache Support.