Hello. I experienced the same problem and was becoming more and more frustrated at the seeming randomness of this bug. I eventually worked out that if I selected all the regions on the track in question and joined them, the resulting audio file was often (but not always) then able to be edited using flex. If that doesn't work, select all the regions on the track and solo the track, making sure to disable any panning or EQ you might have set previously. Then select share, export to disk, select wave format and check the box underneath to share just the selected regions. Once this is done, import the resulting .wav file back into Garageband, line it up on a new track coinciding with the original, and this should definitely work with flex.