Tab order error

On a form we have a series of fields that contain calculations which the user fills in as they tab through. (works fine) These are required fields.

The next section (a radio button) asks a simple yes/no question with a conditional text field underneath. A 'YES' and the user correctly fills in the text field, a 'NO' and they 'should' move to the next section. However if the user 'Tabs' from a 'NO' answer then the Tab action takes them to the previous field and not the next field.

If the same process happens using the 'NEXT' label indicator then the order is correct and the user is moved through the form in the correct direction.

This happens in two separate locations on the form - both times the user is taken to the first field even though the fields are completed.

Anyone had a similar experience

Thanks

John