In general, in dynamic blocks with hatch, make your hatch boundry a closed polyline, even if you have to put it on defpoints to hide it, then don't select the hatch when you select the stretch objects (more specifically, don't select the center node of the hatch), else it wont stretch, it will just move.
BUT.. if you use a flip or a rotate action, you do select the hatch.
Of course, make sure your hatch is associative and you will have much better results if you select the polyline instead of selecting the area.