Since the flooring isn’t permanently fixed to the subfloor, it can shift. A dip in the subfloor can be one reason for this. The best way to counteract this is to use a transition strip of molding fill.  Simply fill the gap between the floating floorboards and the subfloor. Once you’ve added a drop of construction adhesive to the gap, slide the strip into place. This will go a long way toward keeping the floor in place.