As you say, it is supposed to rotate so that it lays flat on any thickness. However, it should not rotate such that it does so easily when the slideway is raised and flops over to the right, if that is happening try lifting the black handle up all the way and rotate the slideway one full turn clockwise then return the black handle down to its normal position and see if it flops over still. If it does repeat the process (although this is unlikely).