VNAV and saved flight

I do normally setup the FMC and save the flight before start engine and then open it when I have time to fly but with the NGX the VNAV don't engage once in clb, it's like is resetting altitude calculation done before, but on the ground all altitude are set and check.The only solution is to change the flightplan like runway or waypoint even in the air and then the Vnav works fine.I don't know if it's a bug but it's a strange behave.IMHO