Satnav destination screen icons greyed out

Intermittently, when I press the Destination button by the SatNav screen, all of the option icons for entering the desired destination are greyed out (not selectable) except for the Map icon. Sometimes it is OK and other times they go fully bright (ie selectable) if I wait. Similarly with the Menu options (for editing/adding memory points etc.) Seems to be something to do with the SatNav memory. I never know whether I am going to be able to use the satnav or not.

Any ideas?. I have replaced the map DVD recently, but it has worked OK since and I can't see why this should cause it anyway.