Thats happening cause the cables are used to being in the position the were in the box. Moving the cables wont do anything. Two things that will help, tighten the nut that holds the highback to the frame. Extra friction between the highback and frame will help resist the highback coming back up. Second is to fold down the highback at home and store the board upside down for a while in a way that completely extends the highback, basically breaking in the cable.
That thing I do to escape the world and have fun with my friend