I had a similar problem with my Walther Nighthawk. Even trying different brands of caplets including the lubed Walther ones didn't help - I thought the seal had gone.

A Very Nice Chap at Bury Gun Room told me that the brass screw seal on the drop-out CO2 mags have a tendency to work loose, and to try tightening the screw before fitting new seals, and it worked a treat!

If you can get to it on the PPK, give it a go first.