westinghouse,
I think you have to be within 1.5 NM of your Final fix for for the Approach Nav. or Final mode to engage when performing a managed non precision approach following radar vectors. If the vectors you are given do not take you into that zone Nav mode will not engage, however I do not know why it seems to work with the old Honeywell FMGCs as you described.
What you can do though, is to insert a direct to your final fix with the radial inbound function, when given radar vectors for a managed non precision approach. In this case the aircraft will always give you a nav intercept and approach nav and final will arm regardless of how far from the final fix your radar vector intercepts the final course. When inserting the DIR TO with RADIAL inbound remember to put the reciprocal of your final course. Hope this helps,
fly safe!