Messed around with this for an hour and checked out multiple articles. I have an Assembler and a Refinery that are one block apart from each other. Installed a Conveyor Tube between them but I can't get it to work. It stays yellow. What am I missing here?

I'm connecting the tube on the smaller yellow squares. Not the large ones with the handles. Interestingly, when I first did it, I connected the tube to the large yellow squares and the tube light was green.