There are possibilities why it is going wrong. So how do you show the form on the live site? With the on-line form webpart or in the content? Which Kentico version are you using?
You can also look at the configuration, for example do you use an custom layout or standard. That's maybe the reason why it not showing your new field.
You can read some documentation and see what is different with your situation.
Here is the documentation of forms : https://docs.kentico.com/display/K82/Forms
And here how to place the form on your site : Displaying forms on the live site
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