Ajax validation is not working
If the AJAX validation is not working for your form, please double check the JS scripts that are being run on the page (custom scripts from the CSS and Javascript tab) and make sure there are no errors in the syntax. JS errors can also be generated by 3rd party plugins, if you check your custom scripts, but the issue still persists you could try going to the Plugin Manager, disable each extra plugin one at a time and test the validation again.
Another cause might be your template, if you are using a non-commercial template, it may not load all the necessary tools in order for the AJAX validation to function properly. You can easily test this by viewing the form in a default Joomla! template (ja_purity, beez) and see if the validation works.
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