2 things firstly pop to my mind,
Check your surface normals
Make sure they can be attached successfully.
By that last thing I mean if one surface has only a certain number of spans etc, and the other has more or less, there could be issues like you are having.
Mike has a great tutorial regarding attaching/stitching sufaces like this in the VIP section called "Arm Stitching", so you might wanna check it out
Otherwise, a text version is at his site as well.
I believe his tutorial site is in our tutorial links database page, so check there first
Israel "Izzy" Long
Motion and Title Design for Broadcast-Film-DS