Where did you purchase them from? I've owned two of the 8 Liter and now eight of the 20 Liter and I've had to warranty one of each of them for the same problem. That said, I had one that was leaking, and I was able to get it to seal by doing the following:
Remove both the yellow and red posts. Using a 10 mm socket, remove the innards, inspect and clean and reassemble. Reattach the two posts in the opposite orientation and tighten by hand as tightly as you can.
Now carefully inspect the sealing edge of both the keg itself and the tapping head for defects. If they look fine and it still loses gas over time, then they should be warrantied.
Also, when you do your test where they "go soft", is that with already fully carbonated water in there to begin with? For example if I take still tap water and put it in a keg with 20 psi on the headspace, that small amount of CO2 is simply going to dissolve into the water and the keg will appear to be flat. It needs to be under pressure for a full week or better before you begin your "test" by removing the source of gas.
Also, if you leave a pressure gauge hooked up to them, then beware that the included orange o-rings are not the best. I have had to replace a few of mine with standard corny o-rings which are a touch thicker and frankly also probably more durable.