Your seal has become compressed and is no longer big enough to tightly seal the chamber. Whenever the seal is inside the chamber, it is being held compressed. Eventually the compression forces prevail and the seal becomes too small. To maximize the life of your seal you need to minimize the time your seal is held compressed. This means eject the spent coffee immediately after every pressing and store the seal either pushed all the way through or removed from the chamber.
What can I do?
Buy a new seal: The seal can easily be replaced on the end of the plunger so one solution is to buy a new seal by calling our Customer Service Department at 1-650-493-3050 (if you live in the USA) and by contacting the importer/distributor in your country (if you live outside the USA). With care, a new seal should last at least three years.
For a quick fix: Joe Lindsay sent us his short-term fix: First place the rubber end of the plunger in some hot water for a couple minutes. Then press the rubber end of the plunger onto a flat surface such as a cutting board. While pressing roll the seal around on its edge so that you are pressing the edge out, widening the circumference of the seal.