Captcha image issue on lost login - count(_SESSION) == 0

Hi
I've just moved my new J2.5 site from test domain to live and noticed that the Captcha image is showing on the registration page but not on the forgotten login page.

Registration page working here
alumni.chevron.com/component/comprofiler/registers.html

Lost login click to expand and image not showing
alumni.chevron.com/forgotten-login.html

If viewed in Chrome you can actually grab the image url and looks ok but when load that image I see a white screen "count(_SESSION) == 0"

I've seen a similar issue mentioned in this forum years ago and seems to be an issue with captcha not being able to read the configuration.php and get the session but why would it work on registration but not on the lost login?

Any ideas please

Thank you in advance.

Only difference between test and live was that I enabled the caching and css/js compression in the Joomlart JEdenite template and disabling caching seems to get it working so must have been a caching problem as was also getting same code each page load so have switched off template caching for now