What you have is a "cargo rack", not really a good bike rack and that is the problem.
Found a pix that should look pretty much what your cargo rack looks like..

There is no way to keep the bikes from moving around or rubbing and trying to tightly strap them into one big bunch is going to result in what you are seeing, damaged bikes.
You need a bar like a bike rack that goes all the way out to the end of your cargo rack that has two mounting points for each bike.
Like this..

Each bike is tied at two points and cannot touch any of the other bikes.
Each bike must have space between the next bike.
You do have a bonus with that cargo rack, you can bungie the wheels of each bike down to the cargo rack. That insures there is zero movement of the bikes and they can't swing back and forth.
To use a rack like the one in the pix above you need either a hitch receiver installed on your cargo rack or some modification to the floor of the rack to allow mounting the vertical part of the rack to the cargo rack floor..