The biggest issue with tough doors is an uneven door frame. If the ground underneath the door frame is uneven, or the container is loaded heavy to one side, or on a surface that causes it to bend or twist, the doors won't open and close properly. Container doors are built to be opened and closed when the container is sitting squarely on a chassis. Over the course of 20' or 40', it's possible for a container to twist just enough for the door frame to be out of whack. Placing wood blocks under the door frame at the corner posts is the best way to solve that problem before it starts. You can also use an industrial lubricant on the hinges and work the doors back and forth for a few minutes to loosen them up.