Chances are the fuel tank is stuck and just needs a little push to loosen it. Put a jack under the fuel tank and a piece of wood such as a 2x4 between the jack and the plastic tank. Don't use the jack without a piece of wood or something to spread the pressure. Otherwise you will end up popping a hole in your plastic fuel tank. Gently jack the fuel tank up until you hear it crack loose. Lower and remove the jack and you should be able to get the tank out.