Help: Why do I occassionally have zero head on kegged beers?

[macaronijones]

I will later this weekend. To be clear, carbonation wasn't prevented, it occurred at high psi (30) and then was lost over the course of a week after it was dropped to serving pressure (12). I've never taken apart a QD or gas post so not familiar with the components, but after playing with the QD and listening to the leak it seems like the poppet didn't "seat" perfectly or whatever and allowed gas to slowly escape from the keg, while somehow not simultaneously draining the C02 tank. Honestly I can't quite decide how that makes sense, but I can't argue with the results.