The problem is that the original ones were made from real rubber. It's plastic now, and they don't last. I'm quite sure our favorite dealer is doing what he can to get the best quality, but I'm hearing so many people complain. It must be uphill.
I've adapted a set of these to my rack as part of my ongoing restoration project. They don't look original and interfer slightly with the chassis when fully extended, but they seem to be much tougher. I haven't actually driven the car with them yet.
https://www.autodoc.dk/skf/7614714