motor mover wont stay engaged on one side

i have a set of enduro movers on my van, i have a problem that 1 of them wont engage properly, it looks as though the cam is not pushing the mover far enough forward. if i push the arm forward and down it locks onto the shaft and stays there till i disengage them the gap between the mover and tyre is 20mm as per the instructions so my question is is the cam adjustable if not do you think if i filed out the slot a couple of mm deeper with a rats tail file and ground a couple of mm of the heel of the arm this would fix it. any ideas please