Constraining Splines

I have been trying to do some surfacing in SWx 2008 SP5.0. I was in the process of fine tuning the surface by editing the splines used to create it when I noticed the surface was not respending to the changes. Turns out the spline wasn't acting properly. I have an intersection curve and a spline which are constrained to be tangent. When I created the constraint, the spline did as it should in moved into tangency. If I try to change the spline by lengthening or shortening the intersect curve the tangent constraint does not remain in force. It's still there on the list, it just isn't active (or something??) If I delete the tangent constraint and re-create it, it works -- once. Why would a constraint only work when you first create it? Or is there some other way to apply a tangent constraint to a spline?

These are images illustrating the problem. First is the original geometry, then after editing the intersection curve, finally after deleting and re-creating the tangent constraint;