Green spinner - forms stuck on loading

Do a network trace or use the browser dev tool to see http request, are there anything failing?

First thing you can do is add the ?_debug=1 query string parameter to your Form to enable debugging. You will see a “show log” button in the upper right-hand corner of your browser window.  This will give you insights into what may be causing the hang up.

You can read more about it here: Debugging Forms

Also, take a look at your form, you can go into the rules and start a process of systematically disabling the rules in the Initialize or Initalized events to also narrow it down, but start with the debug option as it will give you insight into the order of operations that is happening when you form loads.