ensure all mandatory custom fields are arranged as high up in the stack as possible in the "field customization" section of the "item" module settings.
navigate to settings > items > field customization
click "change order"
drag and drop all your required fields to the top of the unlocked stack of items
You may drag by holding click anywhere on the row of the desired non-locked item,
(icon for unlocked items are shown below)
click "save reorder"
You may now check to ensure all fields show up as needed when you create or edit an invoice