Sometimes the pins in the phone (little gold prongs) get a little dirty. Try opening the phone to see the sim contacts and check for dirt/flattened/broken pins etc. If they are too flat you can try lifting them up (gently as they are fragile) with a small toothpick/tool. Then, you can try cleaning them with an electronics cleaner (something high in alcohol content so won't leave residue). Let us know if this helps