Unable to save form layout when using 'Create new field' section.

This article discusses possible troubleshooting scenario when user encounters the following:

• New field is not saving to form
• They are using Configure Form Layout and adding new field using the 'Create new field' section
• When they hit 'save' nothing happens, and their new field is not saved.
• The new field cannot be found even when searching the table's columns.

This can happen when the user is trying to add a field whose name already exists on a parent table of that form/table.  For example, if they added a custom field to [incident] and are now trying to add the same field name to [task] it will not work.  Although this can appear to be failing silently from the front end, you should be able to see an error present in the node logs when trying to save:

WARNING *** WARNING *** 'u_field_name' is already defined on 'other_table'

This happens because 2 fields cannot have the same name within the same table hierarchy. 

Try adding the field with another name, or remove the existing field first from the conflicting table in the hierarchy.  This is expected behavior.