No "6x9" backs are 9 cm wide. The largest they get is about 56x84mm image area - technically "6x9" is really 2.25" x 3.25".
What problem are you having? The later Graphic backs (lever wind) have extra pin rollers that are supposed to aid with film flatness. The Horseman backs have, I think, larger main rollers. However, the Horseman backs are also a little larger side to side and they may not fit some cameras. There are also earlier types of Horseman back that look a bit different and that don't fit a Graflok-type back at all, so do some research ... a lot of research. I have no experience with Linhof or other makers' backs, but generally the same issues apply.