To Official FAQ has an entry for this problem. The most frequent cause is some plugin or theme causing incompatibilities, for example when the plugin or theme adds different versions of libraries used by the plugin.
This entrance in the plugin knowledge base reports exactly the problem you are having and suggests an article with tips on how to find the real cause of the problem.
Basically the first tip is to change the theme and see if it works. If it still doesn’t work, disable all plugins except Contact Form 7. If this solves the problem, re-enable one by one until you find out what the cause is.
Just to give you an idea, this support topic reports a user’s experience with the same problem. In the end, he found that the plugin WP Facebook Open Graph was the cause.
The problem may occur because you may have different widgets in posts single or because some plugin or theme makes distinction using is_singular() or some other internal Wordpress function that checks the type of page being displayed.
Solving the problem is definitely another story. After identifying which plugin or theme is guilty, you would have to identify exactly the root of the problem.
The most common case, as I mentioned, is to have different versions of the same library as jQuery. You can also look in the HTML source and look for this kind of common problem.
Once you have identified the root of the problem, you have to see if there is an easy solution. In the case of duplicate libraries, the first step is to remove one and see if everything still works. If it doesn’t work, some of the plugins will have to be adjusted to work with the different version.
Also check that all plugins are properly updated. Older versions may have incompatibility with newer libraries.
Have some plugin or code running only on
single.php, maybe comparing html is easy to discover.– brasofilo