Can't connect via docking port between subassembly and main rocket

[jpinard]

I have a nice little rover I made and saved it as a subassembly.   I then built my main rocket and brought in the subassembly but whether I use a docking port or a stack separator I can't get them to connect together.  Any idea why this may be?

[Geonovast]

Place the sub assembly in the editor so it's red/greyed out, aka not attached.

Use the re-root tool to change the root of the sub assembly to the part you want to attach it by.

Grab the sub assembly and attach.