Hi Stephen,
No I don't think you are doing any thing wrong. I too have the same problem. The method used on our trailer to clamp the docking arms to the trailer is not very good. With any amount of wind or cross tide the arms are just pushed down by the boat. It does not need much force.
The docking arms are only useful for locating the position of the trailer once it is in the water. They don't seem to be strong enough to physically restrain the boat.
I would like to replace mine with a stronger version, perhaps using something like 40mm square hollow section steel that slots into a welded bracket on the trailer, but I have not got round to doing anything about it yet.
Regards
Graham Ebb
235/25 Blue Jazz