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I did some test submissions from your form, and it seems not to be showing the Default Thank You Page, nor from the custom URL or direct link.
I did clone your form on my account and the cloned version shows the the Thank You Page. I would recommend you to try making a clone of your form and let us know if the cloned version works. http://www.jotform.com/help/27-How-to-clone-an-existing-form-from-your-account