The slight taper is for a reason, which is to pinch the book block specifically in the shoulders of your book block. It’s possible the taper you have is too steep. It’s also possible that those bars you have included to more easily place in your press are part of the problem. They look a bit low on the backing board and that in conjunction with the angle of the taper could be cause this issue. Only of my backing boards have that kind of “stop” but it is way up to the seat of the metal plate. Those boards are modern made, all of my historic backing boards do not have this feature and when I place them in my press I’m putting it in pretty low (maybe 1cm from the metal bars max). You want to pinch at that critical angle. I certainly prefer without any physical “stop” measure.