problems welding vertex in the uvw editor-3ds max

Hi all, i m currently working on a character in 3ds max 2012, and i have some problems when i'm going to weld vertex in the uvws ediitor, when i move a vertex, its like it tries to auto locate in different vertex positions and when i finally move it to the position i want, i doesn`t weld and the edge disappears. i dont know if iwas clear with the description, but if someone knows how can i fix it, please help me
thanks