Thanks for writing around! Please open the form designer tab and see if there fields added in there, if you don't see any field please add some. Also see https://www.convertplug.com/plus/docs/how-to-use-form-builder-in-convertplus/
If that's not the case then there could be a conflict, please test for the following:
Ensure everything is up to date according to our version compatibility list at https://theme.co/apex/forum/t/troubleshooting-version-compatibility/195. Please follow the best practices when updating your theme and plugins. See https://theme.co/apex/forum/t/setup-updating-your-themes-and-plugins/62 for more details.
Clear all caches including browser cache then deactivate your caching plugins and other optimization plugins.
If you're using a CDN, please clear the CDN's cache and disable optimization services.
Test for a plugin conflict. You can do this by deactivating all third party plugins, and seeing if the problem remains. If it's fixed, you'll know a plugin caused the problem, and you can narrow down which one by reactivating them one at a time.
Let us know how this goes!