You may need to drop a segment in a Timeline and make sure it is aligned with another segment siting on a layer below so you can create a vertical compostion.
An easy way to make sure that the segments are properly aligned is to use the CTRL modifier of the Snap Closest mode.
By default, Snap Closest will snap the segment you are manipulating to the closest Mark, Positioner or Transition on the layer you are dropping the segment on. However, if you hold down the CTRL key while you are moving the segment in the Timeline it will considered all the Transitions present on the layers below.