Locking fields once status changes not working

I have a business rule on a Purchase Order that states:

If Status Code = Paid  -- (Note:  I added a custom value in the Status Code field for Paid)

<lock all 20 fields I listed one-by-one because there's no other way to do it>

Else If

Status Code is not equal to Paid

<unlock all 20 fields I listed>

This rule's scope is set to the form.  When I change the Status Code absolutely nothing happens.

Geez, I just wish something would work -- the first time.