Go back to '1' mode and check the vertices at the edge closest to where the smoothed mesh goes in. Single click on the vertices (dont drag select as it will select all vertices that share that space, we are looking to see if there's more than one at this point) and move it up or down, does it leave part of the mesh behind or does it move as a whole? Undo the move action so the vertex returns to it's original position.If it sperated then merge the vertices,should cure the problem.
Sorry, just looked at the date of your post! You've probably already sorted it by now
