NAV/ADF display on ND

[Paul Harmer]

Hi there,

Probably something very simple and basic that I'm missing :)
but the VOR 1/2 and/or ADF frequency readouts on my 737 ND are
there sometimes and aren't the next time I fire up the system.

Any ideas why they keep appearing and disappearing? I don't
think I'm doing anything different between flights. In fact,
it's the same flight with the same FMC inputs as far as I can
tell.

Thanks,

Paul