In my SharePoint 2013 list I created a field to be allow "Multi-line Rich Text". In the Nintex Form when you click on the field to enter data the Formatting Ribbon controls do not display. I can only add plain text.
usually, as you said, once you focus the rich text field you should see the format text ribbon:
Is it possible to remove the form from the list and check what plain SharePoint shows you? If a default form also hides the rich text ribbon I suggest you re-check the configuration of the column. Before you remove the form you should do an export of it to recreate it later.
If plain SharePoint shows the formatting ribbon to you the problem seems to be inside Nintex Forms.
What happens when you remove the multi line control from your form and re-add it from the list columns section of the form controls section?
Any change in the behaviour?
Retrieving data ...