The problem you are having with the pockets is due to the fact most antiques, the slate does not overhang the frame enough to attach the tabs to. check this to be the case first. With the slate centered on the frame,check to see if there is any overhang to attach the tabs to. make sure the pocket would hang fairly straight and accept balls.
If not, you are going to have to attach some of the tabs to the slate frame in the face of the pocket. we have documented photos showing this was the recommended way. They would attach the bed cloth first then tack the center four tabs to the face with small tacks then install rails and tack remaing tabs to the rails.
one thing we recommend is to back cut the ends of the tabs that are to be attached to the face frame. starting 1/2 inch back from the end of the tab slice the thickness of the tab to it becomes paper thin. this makes a nice transition instead of having the thick ends of the leather stopping abruptly. do this before hanging the pockets!