"Cannot drop here" while trying to put multiple "Router" snaps as "Diff"'s outputs

While trying to arrange several snaps with multiple inputs (e.g. Router) as outputs for a snap with multiple outputs (e.g. Diff), I get the following message and cannot continue.
“Cannot drop here
Drop in another blank area. Snaps cannot be dropped here because they collide with other Snaps.”
Can anyone suggest how to enlarge or rearrange the snaps differently to get the desired effect?