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Unfortunately our lightbox forms are not optimized for mobile devices.
I am creating a feature request ticket and forwarding it to our backend team. We will get back to you as soon as we have any update from them.
For now, I would suggest you to please take a look at the following thread where one of my colleague have shared a workaround to make lightbox form mobile responsive by injecting custom css code: http://www.jotform.com/answers/686728-Lightbox-embed-code-is-not-mobile-responsive