I had the opposite problem. I put my overnight gear in the box, locked it, and when I went to open it, the key wouldn't unlock the case. Fortunately I was away for only one night.
When I got home I dismantled the case and examined it to see what the problem was. It turned out to be interference between the case and the slides that unlock the case's lid. I filed a few of the parts so they moved more freely and it works perfectly now.
I can't remember if I needed to remove the tumbler to open it, but I do have some photos (below) of how this is done if any one needs to know. You can make a release tool from a paper clip, and insert it into the hole above the tumbler. Push down and pull the tumbler out.
Special Tool for Top Case Tumbler Removal

Process for Top Case Tumbler Removal
