There are some type of fields that are not included in the navigation of field to field on the iPad.
These include Drawing, Radio Buttons, and Checkboxes. As you navigate using the "<" and ">", when you get to one of these fields in the field order, you will then need to tap on the field.
This is due to the fact that these type of fields do not have an input method that pops up at the bottom of the form and this is where the Navigation Icons are Located.