This can occur for a number of reasons and is usually due to a plugin or theme action that's either causing a conflict or preventing a script from loading correctly.
If you experience this, the first thing to do is to disable any plugins that you're happy to disable. There are a number of "rogue" plugins (so to speak) that are coded in an "unfriendly" way such that they conflict with neighbouring plugins. We've taken great length to ensure that the Thrive Content Builder complies with the Wordpress Codex, however we can't vouch for third party plugins.
Once you've done this, the next thing to do is to look for error messages in the console. You can do this by trying to edit a page with the Content Builder and hitting f12. This will load up the console where you can see if there are any script errors. Take a copy of anything you see.
Finally, send us a ticket to the support desk with a clear description of the problem, temporary wp-admin details and a copy of anything you found in the console. We'll go ahead and see if we can resolve the problem for you.