Caller display problem.
 

On both mine and my wife's mobiles (both Nokias, but different models) when we receive either an incoming call or a text from someone in our contacts, sometimes the name is displayed and sometimes the number. This seems to be completely random, or at least I can't work out any sort of pattern. For example, if I am rung on my mobile from my home phone, sometimes it will appear on the display as "Home" and other times the number will be shown.
Does anyone else have this problem or have any idea how to solve it?
Thanks.