IF, notice the big IF, the brackets on the trailer frame are aligned directly above the attachment point of the chains on the lever arms, the brackets should not slide. I suspect the brackets were not in exact alignment. When aligned correctly, the brackets should stay put with moderate torque on the square head bolt. When I installed the brackets I had about 3/4" between the bracket and the frame. I put a square of 5/8" plywood in the gap and tightened the square bolt down on the plywood. This prevents the frame dent and is enough pressure to keep the bracket in place.
2009 FF X-189 FDS
2011 Chevrolet Z-71 LTZ 4X4 p/u.
Reese WD hitch
Patient and understanding wife