There are several reasons that your cushions may not rebound as good as they should. Their age is the main reason. When they get old, they dry out and get hard. It happens gradually and may not happen to all six cushions at the same time. Once it starts, it is time to replace all six. The cushions can become separated from the rail liners. This can usually be fixed.  The bolts that hold the cushions on the table may be loose or damaged. This can usually be repaired.