Cover - spine won't snap to front cover

Hi, I have spent two days trying to set up a cover and spine for Amazon KDP and have to admit defeat, so I’m hoping you can help. I have watched 10 videos, read all the online content I can find and am working to Amazon KDP’s cover calculator.

My book is 152.4 x 228.6 mm (6 x 9 inch) with 150 pages. I created a new print document with three pages. Document setup (screenshot at end of this message): facing pages unticked, margins: 3.17 and bleed 3.17 (top, bottom, left and right). Then I selected all three pages in the pages panel and unticked allow document pages to shuffle, and unticked allow selected spread to shuffle.  

Under liquid page rule, I selected scale, then used the page selection tool to select the spine. I then went to Layout/Margins and Columns and set all to 0. Then I went to the control panel and changed the width of the spine to 8.58mm (as per KDP calculator for my books size and page number). This created a space between the spine and the front cover (see screenshot below).

Following instructions, to remove the space, I selected the right  handle of the spine, with the spine page selected, held down alt and dragged the spine to the right. (Alt is the windows command). Per instructions, this is supposed to snap the spine to the front cover without resizing the spine. Instead, and this is my problem – it drags the spine out to its original size. This shouldn’t happen. Any clue as to where I’m going wrong? I went right back to the start and selected re-centre in liquid page rule but that didn’t work either.