Over the years, the piston moving through the seals gradually wears the seal material away, allowing a gradual seepage of wax out of the cylinder. This reduces the pressure within the cylinder and needs to be replaced. But it could also be that the wax in the cylinder just needs a simple kickstart and a warm bath. Here is a tip to see if you can get them working again:
Remove the cylinder and place it in the refrigerator, not the freezer, for 30 minutes. Immediately after removing them from the fridge, run the cylinders under hot water in the sink for 5-10 minutes.