Most likely it is a Graflex back. On the Graphic the roll film back has a rail and the camera a groove. If it is a Graflex rather than a Graphic back, it has a goove instead of a rail and there is nothing to prevent it sliding. One solution you could try is to put something into one of the grooves so they well lock together when the back is fitted.
The other possibility is that someone ground the rail off the back at sometime (To fit the back on a strange camera?) in the past. I have no idea how you could fix that.
Anyway, if the rail on the Graphic back fits down into the grove on the camera, the back can not slide.