Strangely not all of them have lost there strength. You WOULD think since they were all built at the same time they would all be in the same condition...
The loss of strength is related to the number of cycles they have been put through (opening and closing the deck lid). An OEM one sitting on the shelf will be just as strong today as it was when manufactured.
Just like a rubber band, the more times you stretch it and relax it, the weaker it becomes.