While creating an end weld in Creo Simulate, the check box Extend Adjacent Surfaces is not selected by default.

Consider the shell pair model as shown in the following figure.

Create an end weld between surface a and surface b. Do not select the Extend Adjacent Surfaces check box.

When you mesh the model the compressed model is as shown in the following figure.

If you select Extend Adjacent Surfaces meshing fails because the extended surfaces adjacent to surface a do not intersect the target surface b on the other side of the weld.

Create another weld between surface c and surface b. Select the Extend Adjacent Surfaces check box. After meshing, the compressed model is as shown in the following figure:

In the case of this end weld if you clear the Extend Adjacent Surfaces check box, compression fails.