It sounds to me as if you're drawing the B-spline across other lines. those lines would automatically split the B-spline and those lines would prevent weld from welding the spline back together. If you make groups or components of the other lines, you can prevent the B-spline from being split. If you need parts of the B-spline to be inside those groups/components, there are simple ways to do that but I'd want to see an example of model before going into it.