There must be a folded face or hard points or something there. Otherwise, the Mesher would have nothing to seed. Another approach would be to use the Patch Independent tet mesh method. Are you familiar? Basically this will mesh the volume and then projects it to the surfaces. If you adjust the settings a bit, you should be able to have this ignore this thin line/point/whatever it is. To use it, add a Method with Type-> Tetrahedrons and then change the Algorithm from Patch Conforming to Patch Independent.