Help Contact display on screen

If anyone can help me that would be great. I have a HTC Incredible S.
Whenever someone from my contacts list, EG: "mum" calls me, it comes up with "unknown".
How do I change settings so that when "mum" calls me, "mum" comes up on screen so i know to answer.
I had all my contacts saved to sim. I changed this so contacts are now on phone, but it still comes up "unknown".