Hi,
I'm not sure if this has changed with the newer Nokia's but when I had one and had the sim and handset memory both in use it could confuse the phone and sometimes not show the callers/texters name. Look in your settings and choose one memory.
If that's no longer the case then another option is to delete the number out of your contact list and call lists/history and re enter it perhaps with a full stop after the name just to slightly change how it was entered before.
If no joy then give o2 a call to check your account and perhaps a soft reset {battery pull} of your phone may help.
Or as a last resort a hard reset.
Make a back up first to save your data.
How to factory reset:
Warning! All data will be lost!
1.Power on your phone.
2.Type on keyboard *#7370#.
3.Enter security code 12345.
4.Wait until the phone restarts.
