Add Force expects to be called every frame. If you want to yank it bit-by-bit, Add Impulse instead. If it does not budge, tick Vel Change to test it out (this will ignore mass). If it works, untick it and adjust the force needed.
Another thing is the shape of the collider. If you’re dragging a cube over craggy terrain, it may snag. If you are, indeed, dragging boxes, it may be a good idea to make the collision more forgivable; a bevelled edge like this can work really well:
18 DOP will be fine in most scenarios.