This a cpvc project, right? Multiple timelines are opened when necessary, and it is indeed not possible (like in cptx projects) to change the stacking order. However this could be a workaround: move the timeline that you want to go down horizontally until you have an empty space: you can then move it down. You will have to move the other timelines up in the location where you want to have this object at the bottom... Then you'll have to move again. Very cumbersome, I think it is much easier to reflect before adding objects, and add them in the correct order. CPVC-functionality didn't get much attention after it was introduced, probably it is not that popular?