There's only a few things that can cause this issue, the retracting pin at the gas door is stuck in or broken, if it's stuck in it's most likely due to corrsion which means the cable need to be replaced, you may be able to get the cable loose with lubricant but I doubt it, or in the best case the tab on the gas door is bend away from the pin simply bend it back towards the pin, but do it a little at a time until you get it to catch, but before you bend it make sure the pin will retract when the door release is pulled or you will have a problem getting the door opened to put in gas, same issue if you bend the tab to far!