I feel like I'm going mad, I'm trying to use the Extrude edges by rails tool, but whatever I do it won't let me pick the rails. I can select the profile which turns blue but when I click on the rails it just keeps asking me to select a rail and they won't turn pink. Though if I select the melding profile that is happy to be selected as a rail and will turn pink. I've made sure to update Sketch up and Extrusion tools to the latest versions. I really want to be able to use this tool, what is going on??? Please help!!

I've just started using the weld extension in sketch up pro 2021. It all seems to be working well and rejoining the geometry when I check it. however once I save the file, close it and re- open it half of the welding hasn't saved. Any ideas Why this is happening?