Fields selectable on forms, but not selectable on one particular form

We have two boolean SFDC Custom Fields that are not selectable as fields on a particular form. However, those fields are selectable options in a newly created form.

The form in question is a clone of an existing form, and uses progressive profiling (however, these fields will be placed outside of the progressive profiling).

What is blocking these fields from being selectable on this particular form? I found discussions on fields not being selectable in forms, but I couldn't find any where just one form was impacted.