If the board nose starts to slip down the mesh then the board could touch the back wheel of the bike. The cause of this is slack in the line between the buckle at the "Y" and the locking tab. The tab and the buckle should be about 8 inches apart (14cm). And the webbing pulled tight. This will lock the board in place.
Extra hold can be achieved by wrapping the extra webbing around the lock tab and tie a knot. This is good for extra really long bike rides.
If the unit is not pulled tight into the frame by the webbing this may cause your wheel not to stay straight and touch your board.