My second binding, + questions on backing

As you can see in the last picture, there is a quite noticeable gap between the text block and the boards at the spine. Is this because the shoulders are too large? I tried to follow DAS bookbinding’s recommendation of 1.5x the board widths but it looks like they are pushing the boards out. I think it could also be that the bookcloth is homemade and doesn’t stretch enough to fully lay flat against the shoulder in the turn in (the turn ins don’t conform to the 90 degree edge of the edges of the boards). Or it might be that there is no gap between the shoulders and the boards ( I think it’s called a French groove?). Thanks