No tab number for some fields in a fillable PDF

Hi there

I've created a fillable PDF form from a Word doc, using the 'prepare' tool. It recognised most fields correctly, but some I needed to change /add to; radio buttons / check boxes / dropdowns. 

When I previewed the form and checked the tab order, it skipped past all of the fields I amended / added; they appear in the field list, but don't have an associated tab number, so just skipped straight over. What am I doing wrong?