Lol.. you must be doing it wrong..
This, tie a knot in the end of the rope so it won't slip through, rescue systems training calls it a safety.I've stood there there and watched it happen with the owner of said bote and said high end dock lines. We swapped the dock line to a cheap one of the same size, no slippage.
There is nothing magical going on, the Danik hook works on friction. Some of the nice custom dock lines slip in the Danik hook, I'm assuming because of the material and/or the weave. I'm just saying this as an FYI.. the hook is not 100% foolproof. Mine have never moved, but I don't use it high end custom lines lines.
You can always tie a knot behind the hook for insurance.