Unfortunately, that is a limitation of Airtable forms — they will only show the primary field.
One workaround that you can use is to change your primary field to become a formula field that displays all the different information that you would like to see.
Otherwise, you’d have to use external form tools to make this work.
Thanks for that info @ScottWorld - at least I’m not doing anything wrong! I’ll have a go at changing the primary field to a formula field and see if that works for me before trying out the external form tools.