On the ends of the twine tubes should be either "bullets" or two flat sprigs in the end of the twine tube. If the end of the twine tube has a notch worn into it or if the flat springs have a notch worn into them the twine can pop back up the twine tube. Remove the end of the twine tubes and inspect. It is hard to see the notch in the flat leaf spring with the twine tubes in place. If a notch is in the tube itself, you can switch the two twine tubes. If the flat leaf springs has a notch, then the flat leaf springs need to be replaced.
I remove all tension at the twine box. There are enough corners in the twine route to provide enough tension.