One side is long, one side is short - match them. If you are, either the plug or the port is damaged. (Look at the plug and make sure it's not pushed together long side to short side. If it is, don't try to repair it - you'll end up damaging the port in the phone. Replace it. (Cables are cheap - look in a dollar store or a pharmacy.
Also look in each one (the plug and the port) and if you see dirt, blow it out. Use a small pointed wooden toothpick to loosen any that won't blow out.