Question x axis gantry get stuck

hi, all been good for a while, but last 2 carvings I have had issues at random points, x axis sticks, sending steppers out of sync, when I send it back it looks like something has dug into the aluminium rail! can sort with a bit of emery cloth, but its happened 3 or 4 times now, and cannot work out whats causing it, images attached, any suggestions appreciated

hi all just to let you know, have solved this, found 1 screw (4 screws in sets of 4) holding the plate to the z axis structure had worked free and sticking out enough to cause this, but rubbing on the rail! difficult to see untill the whole thing was taken off