Tabbing between protected cells
 

I have a document that is a fill in that I have protected the document and
have only certain cells that are available to enter data into. It is setup
so that the user can tab between the fields to enter the data. However the
tab function stops halfway through the document and will not continue unless
you place your cursor in the next available field and then it starts again
and stops in the exact same place going back and forth between to cells. How
can I fix this?