Comments not working #respond not loading

Hi
I have switched on the option in settings/discussion to allow people to posts and on the posts that I have checked the ‘Allow Comments’ field the footer shows the text:
‘You can leave a response, or trackback from your own site.’

‘leave a response’ is hypertext and if you click it it changes the url to the post url plus #respond but then nothing happens.

I’ve seen talk some places that permalink issues may cause this error, but I use the custom structure:
/%category%/%postname%/

Any ideas. I’m very new to WP and php, so don’t be afraid to go back to basics.
Many thanks