Version 3.3.1 – so not TOO old, I don’t think.
If I switch to the default theme do I lose all the settings/content?
I did notice that the other site – the one on which it’s working fine – is a newer version of WP, version 3.4.2. So if I upgrade the one on which it’s not working to a newer version of WP do you think that might help?
If I try that, should I uninstall or deactivate your plugins, first?