Looks like there is way to much separation for that tool. It only works at a fairly small distance I think. One way to fix it is to delete one side. Then select just the edges of the symmetry seam. Then set the size for that width to 0.00. It will be easy to know which dimension to set at zero because it will be the smallest one. After your seam has a zero size width, then create the symmetrical copy and weld the two halves together.