This drives me crazy because I can never remember why this happens and I have to search for the answer. Make sure to Save then Publish! The new column will not show up on the form unless you publish the form.
When testing data in Dataverse and you add a new column, you have to add the columns to the "Main Form".
1.
2, add your column/field
3. Save and then Publish! I always forget the publish part! :(